Bash Reference Manual

This text is a brief description of the features that are present in the Bash shell (version 4.1, 23 December 2009). This is Edition 4.1, last updated 23 December 2009, of The GNU Bash Reference Manual, for Bash, Version 4.1. Copyright c 1988–2009 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover texts being “A GNU Manual”, and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License”. (a) The FSF’s Back-Cover Text is: You are free to copy and modify this GNU manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom.” Published by the Free Software Foundation 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

1 Introduction
1.1 What is Bash?
Bash is the shell, or command language interpreter, for the gnu operating system. The name is an acronym for the ‘Bourne-Again SHell’, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell sh, which appeared in the Seventh Edition Bell Labs Research version of Unix. Bash is largely compatible with sh and incorporates useful features from the Korn shell ksh and the C shell csh. It is intended to be a conformant implementation of the ieee posix Shell and Tools portion of the ieee posix specification (ieee Standard 1003.1). It offers functional improvements over sh for both interactive and programming use. While the gnu operating system provides other shells, including a version of csh, Bash is the default shell. Like other gnu software, Bash is quite portable. It currently runs on nearly every version of Unix and a few other operating systems − independently-supported ports exist for ms-dos, os/2, and Windows platforms.

1.2 What is a shell?
At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions. A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of gnu utilities. The programming language features allow these utilities to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as ‘/bin’, allowing users or groups to establish custom environments to automate their common tasks. Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file. A shell allows execution of gnu commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands’ environments. Shells also provide a small set of built-in commands (builtins ) implementing functionality impossible or inconvenient to obtain via separate utilities. For example, cd, break, continue, and exec) cannot be implemented outside of the shell because they directly manipulate the shell itself. The history, getopts, kill, or pwd builtins, among others, could be implemented in separate utilities, but they are more convenient to use as builtin commands. All of the shell builtins are described in subsequent sections. While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions.

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Bash Reference Manual

Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. Each of these features is described in this manual.

‘(’. A control operator or a redirection operator.. ‘&’. See Section 3. or ‘)’. for a list of redirection operators. name A word consisting solely of letters. ‘. when executing a command. separates words. The value is restricted to eight bits. Operators contain at least one unquoted metacharacter.’. Bash is primarily concerned with the Shell and Utilities portion of the posix 1003. the resulting fields are used as the command name and arguments. Most reserved words introduce shell flow control constructs.Chapter 2: Definitions
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2 Definitions
These definitions are used throughout the remainder of this manual. and beginning with a letter or underscore. metacharacter A character that. A metacharacter is a blank or one of the following characters: ‘|’. It is a newline or one of the following: ‘||’. A command that is implemented internally by the shell itself.
operator
process group A collection of related processes each having the same process group id. After expansion.6 [Redirections]. field A unit of text that is the result of one of the shell expansions. ‘|’. ‘. A set of processes comprising a pipeline. that are all in the same process group. rather than by an executable program somewhere in the file system. page 26. ‘<’.
control operator A token that performs a control function. Names are used as shell variable and function names. A space or tab character. POSIX blank builtin A family of open system standards based on Unix. and underscores. ‘(’. and any processes descended from it. ‘|&’. when unquoted. A string of characters used to identify a file. ‘. process group ID A unique identifier that represents a process group during its lifetime. ‘&&’. or ‘>’. numbers. ‘&’.
filename job
job control A mechanism by which users can selectively stop (suspend) and restart (resume) execution of processes.
.’.1 standard. ‘)’. such as for and while. reserved word A word that has a special meaning to the shell. Also referred to as an identifier. exit status The value returned by a command to its caller. so the maximum value is 255.’.

special builtin A shell builtin command that has been classified as special by the posix standard. token word A sequence of characters considered a single unit by the shell. It is either a word or an operator. A sequence of characters treated as a unit by the shell.4
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return status A synonym for exit status. signal A mechanism by which a process may be notified by the kernel of an event occurring in the system. Words may not include unquoted metacharacters.
.

6 [Redirections]. Reads its input from a file (see Section 3.Chapter 3: Basic Shell Features
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3 Basic Shell Features
Bash is an acronym for ‘Bourne-Again SHell’.5. 2. The rules for evaluation and quoting are taken from the posix specification for the ‘standard’ Unix shell. employing the quoting rules to select which meanings to assign various words and characters.1 [Invoking Bash]. shell functions. and the rest of that line. page 6. Performs any necessary redirections (see Section 3. Parses the tokens into simple and compound commands (see Section 3.
3.1.2 [Quoting].
. These tokens are separated by metacharacters. the shell reads its input and divides the input into words and operators. waits for the command’s exit status. Breaks the input into words and operators. obeying the quoting rules described in Section 3. page 29). the shell does the following: 1. page 33). redirects input and output as needed. 4.1 Shell Syntax
When the shell reads input. which are a way to direct input and output from and to named files. and makes that exit status available for further inspection or processing. Executes the command (see Section 3. shell parameters. expands others. the shell ignores the comment symbol (‘#’). Alias expansion is performed by this step (see Section 6. control structures. shell expansions. 5. 3. redirections. removes the special meaning of certain words or characters. page 17). or from the user’s terminal. page 7).1 Shell Operation
The following is a brief description of the shell’s operation when it reads and executes a command. Basically.8 [Filename Expansion]. All of the Bourne shell builtin commands are available in Bash.5 [Exit Status]. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne.
3. The shell then parses these tokens into commands and other constructs.6 [Aliases].2 [Shell Commands]. page 24) and commands and arguments. page 26) and removes the redirection operators and their operands from the argument list.7 [Executing Commands].1. executes the specified command.5 [Shell Expansions]. page 79). from a string supplied as an argument to the ‘-c’ invocation option (see Section 6. This chapter briefly summarizes the shell’s ‘building blocks’: commands. breaking the expanded tokens into lists of filenames (see Section 3. Performs the various shell expansions (see Section 3. 7. Optionally waits for the command to complete and collects its exit status (see Section 3. roughly speaking. Otherwise.8 [Shell Scripts]. it proceeds through a sequence of operations. If the input indicates the beginning of a comment. and how the shell executes commands.7. 6. page 71). page 32).

‘!’. A double quote may be quoted within double quotes by preceding it with a backslash. with the exception of ‘$’. and double quotes.2.
3. If a \newline pair appears.6
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3. ‘‘’.1.2.2. with the exception of newline. or newline. history expansion will be performed unless an ‘!’ appearing in double quotes is escaped using a backslash. single quotes.4 ANSI-C Quoting
Words of the form $’string’ are treated specially. ‘"’. ‘\’. The special parameters ‘*’ and ‘@’ have special meaning when in double quotes (see Section 3. Within double quotes.3 [Shell Parameter Expansion]. When the command history expansion facilities are being used (see Section 9. page 121.
3. ‘\’.1. The characters ‘$’ and ‘‘’ retain their special meaning within double quotes (see Section 3.3 Double Quotes
Enclosing characters in double quotes (‘"’) preserves the literal value of all characters within the quotes. even when preceded by a backslash.5. ‘‘’. Each of the shell metacharacters (see Chapter 2 [Definitions]. The backslash retains its special meaning only when followed by one of the following characters: ‘$’. See Section 9. page 17). usually ‘!’.3 [History Interaction]. Backslashes preceding characters without a special meaning are left unmodified. and to prevent parameter expansion.
3. it is removed from the input stream and effectively ignored). the \newline is treated as a line continuation (that is. page 19). It preserves the literal value of the next character that follows. and the backslash itself is not quoted. If enabled.1.2. when history expansion is enabled. backslashes that are followed by one of these characters are removed. page 3) has special meaning to the shell and must be quoted if it is to represent itself. the history expansion character. There are three quoting mechanisms: the escape character.1.5 [Shell Expansions]. are decoded as follows: \a \b \e \E alert (bell) backspace an escape character (not ANSI C)
.2 Single Quotes
Enclosing characters in single quotes (‘’’) preserves the literal value of each character within the quotes. Quoting can be used to disable special treatment for special characters.1 Escape Character
A non-quoted backslash ‘\’ is the Bash escape character.1 [Bash History Facilities]. The backslash preceding the ‘!’ is not removed. A single quote may not occur between single quotes. for more details concerning history expansion. must be quoted to prevent history expansion.1. page 123).2 Quoting
Quoting is used to remove the special meaning of certain characters or words to the shell. if present. Backslash escape sequences.
3. The word expands to string. and. with backslash-escaped characters replaced as specified by the ANSI C standard. to prevent reserved words from being recognized as such.

2 [The Shopt Builtin]. possibly adding a suffix of ‘.mo. page 55). the dollar sign is ignored. Some systems use the message catalog selected by the LC_MESSAGES shell variable.
. separated by spaces.3 [Interactive Shells]. as if the dollar sign had not been present. a word beginning with ‘#’ causes that word and all remaining characters on that line to be ignored. More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second.2. If you use the TEXTDOMAIN variable.Chapter 3: Basic Shell Features
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\f \n \r \t \v \\ \’ \" \nnn \xHH \cx
form feed newline carriage return horizontal tab vertical tab backslash single quote double quote the eight-bit character whose value is the octal value nnn (one to three digits) the eight-bit character whose value is the hexadecimal value HH (one or two hex digits) a control-x character
The expanded result is single-quoted. the replacement is double-quoted.2 Shell Commands
A simple shell command such as echo a b c consists of the command itself followed by arguments. in a loop or conditional construct. or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see Section 4. Others create the name of the message catalog from the value of the TEXTDOMAIN shell variable.
3.
3.
3. An interactive shell without the interactive_comments option enabled does not allow comments. Still others use both variables in this fashion: TEXTDOMAINDIR/LC_MESSAGES/LC MESSAGES/TEXTDOMAIN. for a description of what makes a shell interactive.3.1. you may need to set the TEXTDOMAINDIR variable to the location of the message catalog files. page 75. See Section 6.3 Comments
In a non-interactive shell. If the string is translated and replaced.5 Locale-Specific Translation
A double-quoted string preceded by a dollar sign (‘$’) will cause the string to be translated according to the current locale. The interactive_comments option is on by default in interactive shells. If the current locale is C or POSIX.mo’. or in some other grouping.1.

2. An external time command cannot time these easily.. page 51). and pipelines. page 30). If ‘|&’ is used. The ‘-p’ option changes the output format to that specified by posix.3. If pipefail is enabled. The reserved word time causes timing statistics to be printed for the pipeline once it finishes.2. ‘&’. terminated by one of the shell’s control operators (see Chapter 2 [Definitions]. or 128+n if the command was terminated by signal n. It’s just a sequence of words separated by blanks. This implicit redirection of the standard error is performed after any redirections specified by the command.
. shell functions. The TIMEFORMAT variable may be set to a format string that specifies how the timing information should be displayed.2. The exit status of a pipeline is the exit status of the last command in the pipeline. which have equal precedence.] The output of each command in the pipeline is connected via a pipe to the input of the next command. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command’s execution.3 [Command Execution Environment].
3. page 8). it is shorthand for 2>&1 |.7. the standard error of command1 is connected to command2 ’s standard input through the pipe. followed by ‘. or zero if all commands exit successfully. with the rest of the words being that command’s arguments. The first word generally specifies a command to be executed.1 waitpid function. Each command in a pipeline is executed in its own subshell (see Section 3. or ‘||’. page 32) of a simple command is its exit status as provided by the posix 1003.5 [Exit Status]. The format for a pipeline is [time [-p]] [!] command1 [ [| or |&] command2 .7. the shell waits for all commands in the pipeline to complete.3 Lists of Commands
A list is a sequence of one or more pipelines separated by one of the operators ‘.’. If the pipeline is not executed asynchronously (see Section 3.1 Simple Commands
A simple command is the kind of command encountered most often.2 [Bash Variables].8
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3. page 61. That is. Of these list operators.’. the pipeline’s return status is the value of the last (rightmost) command to exit with a non-zero status. ‘&&’. the exit status is the logical negation of the exit status as described above. See Section 5. This connection is performed before any redirections specified by the command.’ and ‘&’.2. or a newline. The use of time as a reserved word permits the timing of shell builtins. and optionally terminated by one of ‘. The return status (see Section 3. The shell waits for all commands in the pipeline to terminate before returning a value. If the reserved word ‘!’ precedes the pipeline. for a description of the available formats.. page 3).2 Pipelines
A pipeline is a sequence of simple commands separated by one of the control operators ‘|’ or ‘|&’.3 [Lists].
3. ‘&’.1 [The Set Builtin]. each command reads the previous command’s output. unless the pipefail option is enabled (see Section 4. ‘&&’ and ‘||’ have equal precedence.

2. page 26) associated with a compound command apply to all commands within that compound command unless explicitly overridden. command1 returns an exit status of zero. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator.6 [Redirections]. it may be replaced with one or more newlines.
3. and and or lists are executed with left associativity. done
. and the return status is 0 (true).4. When job control is not active (see Chapter 7 [Job Control]. page 89). and and or lists are sequences of one or more pipelines separated by the control operators ‘&&’ and ‘||’. do consequent-commands. respectively. or zero if none was executed. is redirected from /dev/null. the shell waits for each command to terminate in turn. do consequent-commands. Any redirections (see Section 3. If a command is terminated by the control operator ‘&’. Commands separated by a ‘. until The syntax of the until command is: until test-commands. conditional commands. the standard input for asynchronous commands. the shell executes the command asynchronously in a subshell. command1 returns a non-zero exit status. and only if. while The syntax of the while command is: while test-commands. Note that wherever a ‘. Bash provides looping constructs. This is known as executing the command in the background.4 Compound Commands
Compound commands are the shell programming constructs. The return status of and and or lists is the exit status of the last command executed in the list. and mechanisms to group commands and execute them as a unit. and only if. The return status is the exit status of the last command executed. An or list has the form command1 || command2 command2 is executed if.1 Looping Constructs
Bash supports the following looping constructs.Chapter 3: Basic Shell Features
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A sequence of one or more newlines may appear in a list to delimit commands.
3. in the absence of any explicit redirections. done Execute consequent-commands as long as test-commands has an exit status which is not zero. The return status is the exit status of the last command executed in consequent-commands.’ appears in the description of a command’s syntax. An and list has the form command1 && command2 command2 is executed if.2. The shell does not wait for the command to finish.’ are executed sequentially. equivalent to a semicolon.

page 78).. the arithmetic expression expr1 is evaluated according to the rules described below (see Section 6.) command-list . and if its exit status is zero.2. and execute commands once for each member in the resultant list. esac case will selectively execute the command-list corresponding to the first pattern that matches word.
The break and continue builtins (see Section 4. An alternate form of the for command is also supported: for (( expr1 .] ] . If there are no items in the expansion of words. the consequent-commands list is executed. with name bound to the current member. for The syntax of the for command is: for name [ [in [words . If ‘in words’ is not present.. and the return status is zero. If any expression is omitted.5 [Shell Arithmetic].1 [Bourne Shell Builtins]. it behaves as if it evaluates to 1. each elif list is executed in turn. then more-consequents. as if ‘in "$@"’ had been specified (see Section 3.2 Conditional Constructs
if The syntax of the if command is: if test-commands. expr2 . The return status is the exit status of the last command executed. The return status is the exit status of the last command executed in consequent-commands. the for command executes the commands once for each positional parameter that is set. The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. [elif more-test-commands.10
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Execute consequent-commands as long as test-commands has an exit status of zero.4. page 16). or false if any of the expressions is invalid.. and if its return status is zero. If ‘else alternate-consequents’ is present. no commands are executed. The return value is the exit status of the last command in list that is executed. then alternate-consequents is executed. or zero if no condition tested true. If the shell option nocasematch (see the description of
case
..].. The return status is the exit status of the last command that executes.
3.4.] [else alternate-consequents.2 [Special Parameters]. the corresponding more-consequents is executed and the command completes. expr3 )) . page 35) may be used to control loop execution.. and the final command in the final if or elif clause has a non-zero exit status. ] do commands.. do commands . then consequent-commands. done Expand words. Each time expr2 evaluates to a non-zero value. or zero if none was executed. done First. If test-commands returns a non-zero status. commands are executed and the arithmetic expression expr3 is evaluated.] fi The test-commands list is executed. The syntax of the case command is: case word in [ [(] pattern [| pattern].

The set of expanded words is printed on the standard error output stream... man | kangaroo ) echo -n "two". parameter expansion. The commands are executed after each selection until a break command is executed. done The list of words following in is expanded. no subsequent matches are attempted after the first pattern match.]. There may be an arbitrary number of case clauses. Using ‘. *) echo -n "an unknown number of". Each pattern undergoes tilde expansion. The return status is zero if no pattern is matched.. and the ‘)’ operator terminates a pattern list.. ‘. parameter expansion... arithmetic expansion. If the line is empty. the match is performed without regard to the case of alphabetic characters.&’ in place of ‘. It has almost the same syntax as the for command: select name [in words .’ causes execution to continue with the command-list associated with the next clause.’. each terminated by a ‘.&’. and quote removal before matching is attempted. do commands. Any other value read causes name to be set to null. The PS3 prompt is then displayed and a line is read from the standard input. esac echo " legs. Each clause must be terminated with ‘... The line read is saved in the variable REPLY. Using ‘.’ causes the shell to test the patterns in the next clause. or ‘. The word undergoes tilde expansion. If the line consists of a number corresponding to one of the displayed words. or ‘. Otherwise. The first pattern that matches determines the command-list that is executed.3. and execute any associated command-list on a successful match. page 55) is enabled. command substitution.
. the words and prompt are displayed again.2 [The Shopt Builtin].’ operator is used. Here is an example using case in a script that could be used to describe one interesting feature of an animal: echo -n "Enter the name of an animal: " read ANIMAL echo -n "The $ANIMAL has " case $ANIMAL in horse | dog | cat) echo -n "four".&’.. generating a list of items." If the ‘. select The select construct allows the easy generation of menus. A list of patterns and an associated command-list is known as a clause. the select command completes.Chapter 3: Basic Shell Features
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shopt in Section 4. the positional parameters are printed.&’. If the ‘in words’ is omitted. as if ‘in "$@"’ had been specified. command substitution.&’ in place of ‘. If EOF is read. The ‘|’ is used to separate multiple patterns.&’. then the value of name is set to that word. if any....’. each preceded by a number. and arithmetic expansion. ‘. at which point the select command completes. the return status is the exit status of the command-list executed. if any..

2 [The Shopt Builtin].]] [[ expression ]] Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Section 6. page 41. When it is used. the conditional expression’s return value is 2.5. tilde expansion. arithmetic expansion. page 78). If the shell option nocasematch (see the description of shopt in Section 4. When used with ‘[[’. Conditional operators such as ‘-f’ must be unquoted to be recognized as primaries.2 [The Shopt Builtin]. for a full description of the let builtin. If the value of the expression is non-zero.. An additional binary operator. the match is performed without regard to the case of alphabetic characters. and displays the name and index of the file selected. The ‘<’ and ‘>’ operators sort lexicographically using the current locale. If the regular expression is syntactically incorrect. the return status is 0. page 24. [[. If the shell option nocasematch (see the description of shopt in Section 4. The return value is 0 if the string matches the pattern. Any part of the pattern may be quoted to force it to be matched as a string. otherwise the return status is 1. is available. ‘=~’. Any part of the pattern may be quoted to force it to be matched as a string.3. done ((.5 [Shell Arithmetic]. page 55) is enabled. The return value is 0 if the string matches (‘==’) or does not match (‘!=’)the pattern. do echo you picked $fname \($REPLY\) break. and 1 otherwise. parameter and variable expansion. command substitution.)) (( expression )) The arithmetic expression is evaluated according to the rules described below (see Section 6. and quote removal are performed.8. page 76.12
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Here is an example that allows the user to pick a filename from the current directory.. the string to the right of the operator is considered a pattern and matched according to the rules described below in Section 3. page 55) is enabled. the string to the right of the operator is considered an extended regular expression and matched accordingly (as in regex 3)). Substrings matched by parenthesized subexpressions within the regular expression are saved in the
.. select fname in *. Word splitting and filename expansion are not performed on the words between the ‘[[’ and ‘]]’. This is exactly equivalent to let "expression" See Section 4. process substitution.1 [Pattern Matching].2 [Bash Builtins].4 [Bash Conditional Expressions].. and 1 otherwise.3. with the same precedence as ‘==’ and ‘!=’. the match is performed without regard to the case of alphabetic characters. When the ‘==’ and ‘!=’ operators are used.

For example.3 [Command Execution Environment].3 Grouping Commands
Bash provides two ways to group a list of commands to be executed as a unit. The exit status of both of these constructs is the exit status of list. expression1 || expression2 True if either expression1 or expression2 is true. ! expression True if expression is false. and are recognized as separate tokens by the shell even if they are not separated from the list by whitespace.7. The element of BASH_REMATCH with index 0 is the portion of the string matching the entire regular expression. No subshell is created.2. expression1 && expression2 True if both expression1 and expression2 are true.Chapter 3: Basic Shell Features
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array variable BASH_REMATCH. and each of the commands in list to be executed in that subshell. redirections may be applied to the entire command list. Expressions may be combined using the following operators. When commands are grouped. page 30). This may be used to override the normal precedence of operators.
.4. the output of all the commands in the list may be redirected to a single stream. The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression. () ( list ) Placing a list of commands between parentheses causes a subshell environment to be created (see Section 3. The braces are reserved words. Since the list is executed in a subshell. The parentheses are operators.
3. so they must be separated from the list by blanks or other shell metacharacters. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression. there is a subtle difference between these two constructs due to historical reasons. In addition to the creation of a subshell. variable assignments do not remain in effect after the subshell completes. The semicolon (or newline) following list is required. {} { list. } Placing a list of commands between curly braces causes the list to be executed in the current shell context. listed in decreasing order of precedence: ( expression ) Returns the value of expression.

When the coproc is executed. When executed. The format for a coprocess is: coproc [NAME] command [redirections] This creates a coprocess named NAME. The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists.1 [Simple Commands]. as if the command had been terminated with the ‘&’ control operator. page 80) named NAME in the context of the executing shell. Shell functions are executed in the current shell context. A function definition may be deleted using the ‘-f’ option to the unset builtin (see Section 4.14
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3. the exit status of a function is the exit status of the last command executed in the body.5 Coprocesses
A coprocess is a shell command preceded by the coproc reserved word. no new process is created to interpret them. The reserved word function is optional. Functions are declared using this syntax: [ function ] name () compound-command [ redirections ] This defines a shell function named name. Any redirections (see Section 3.2. but may be any compound command listed above. This pipe is established before any redirections specified by the command (see Section 3. A coprocess is executed asynchronously in a subshell.6 [Redirections]. page 26) associated with the shell function are performed when the function is executed. it is interpreted as the first word of the simple command. with a two-way pipe established between the executing shell and the coprocess. The standard output of command is connected via a pipe to a file descriptor in the executing shell. the parentheses are optional. They are executed just like a "regular" command. The wait builtin command may be used to wait for the coprocess to terminate. the list of commands associated with that function name is executed. The process id of the shell spawned to execute the coprocess is available as the value of the variable NAME PID. the default name is COPROC. The standard input of command is connected via a pipe to a file descriptor in the executing shell. That command is usually a list enclosed between { and }. The file descriptors can be utilized as arguments to shell commands and redirections using standard word expansions. and that file descriptor is assigned to NAME [1].1 [Bourne Shell Builtins]. the shell creates an array variable (see Section 6. page 26). NAME must not be supplied if command is a simple command (see Section 3.6 [Redirections].2. compound-command is executed whenever name is specified as the name of a command. The return status of a coprocess is the exit status of command.2.
3.
. page 35). When the name of a shell function is used as a simple command name. page 9). page 8).4 [Compound Commands].7 [Arrays].3 Shell Functions
Shell functions are a way to group commands for later execution using a single name for the group. and that file descriptor is assigned to NAME [0]. The body of the function is the compound command compound-command (see Section 3. If the function reserved word is supplied. otherwise. If NAME is not supplied.

or one of the special characters listed below. a number. These variables are visible only to the function and the commands it invokes. Function names and definitions may be listed with the ‘-f’ option to the declare or typeset builtin commands (see Section 4. when using the braces.2 [Bash Builtins]. Variables local to the function may be declared with the local builtin. for the description of the trap builtin. Care should be taken in cases where this may cause a problem. Any command associated with the RETURN trap is executed before execution resumes. Functions may be exported so that subshells automatically have them defined with the ‘-f’ option to the export builtin (see Section 4. page 16). page 35.1 [Bourne Shell Builtins]. No limit is placed on the number of recursive calls.4. page 41).Chapter 3: Basic Shell Features
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Note that for historical reasons. A variable may be assigned to by a statement of the form
. the arguments to the function become the positional parameters during its execution (see Section 3. Attributes are assigned using the declare builtin command (see the description of the declare builtin in Section 4. or a newline. If the builtin command return is executed in a function. The ‘-F’ option to declare or typeset will list the function names only (and optionally the source file and line number. (in which case all functions inherit the DEBUG and RETURN traps). if the extdebug shell option is enabled). Functions may be recursive. page 41). that is the function’s return status. This is because the braces are reserved words and are only recognized as such when they are separated from the command list by whitespace or another shell metacharacter. Special parameter 0 is unchanged. When a function completes. When a function is executed.4 Shell Parameters
A parameter is an entity that stores values. in the most common usage the curly braces that surround the body of the function must be separated from the body by blanks or newlines. the values of the positional parameters and the special parameter ‘#’ are restored to the values they had prior to the function’s execution. All other aspects of the shell execution environment are identical between a function and its caller with these exceptions: the DEBUG and RETURN traps are not inherited unless the function has been given the trace attribute using the declare builtin or the -o functrace option has been enabled with the set builtin. A variable has a value and zero or more attributes. a ‘&’. the list must be terminated by a semicolon.1 [Bourne Shell Builtins]. A variable is a parameter denoted by a name. Once a variable is set. A parameter is set if it has been assigned a value. If a numeric argument is given to return. The null string is a valid value. The first element of the FUNCNAME variable is set to the name of the function while the function is executing. See Section 4.
3.1 [Positional Parameters].2 [Bash Builtins]. and the ERR trap is not inherited unless the -o errtrace shell option has been enabled. Also. It can be a name. otherwise the function’s return status is the exit status of the last command executed before the return. it may be unset only by using the unset builtin command. the function completes and execution resumes with the next command after the function call. Note that shell functions and variables with the same name may result in multiple identically-named entries in the environment passed to the shell’s children. page 35). The special parameter ‘#’ that expands to the number of positional parameters is updated to reflect the change.

page 80).2 Special Parameters
The shell treats several parameters specially. then value is evaluated as an arithmetic expression even if the $((..)) expansion is not used (see Section 3. Positional parameter N may be referenced as ${N}. and may be reassigned using the set builtin command. assignment to them is not allowed. In the context where an assignment statement is assigning a value to a shell variable or array index (see Section 6. or as $N when N consists of a single digit. the parameters are separated by spaces.. page 80).16
Bash Reference Manual
name=[value] If value is not given. parameter and variable expansion. page 23). When a positional parameter consisting of more than a single digit is expanded. the variable is assigned the null string.7 [Arrays]. and local builtin commands. When ‘+=’ is applied to an array variable using compound assignment (see Section 6. command substitution.
3. If the variable has its integer attribute set. the expansion of the first parameter is joined with the beginning
@
. Word splitting is not performed. When ‘+=’ is applied to a variable for which the integer attribute has been set. the ‘+=’ operator can be used to append to or add to the variable’s previous value. These parameters may only be referenced. or added as additional key-value pairs in an associative array. with the exception of "$@" as explained below.1 Positional Parameters
A positional parameter is a parameter denoted by one or more digits. The positional parameters are temporarily replaced when a shell function is executed (see Section 3. which is also evaluated. the parameters are joined without intervening separators. arithmetic expansion. value is expanded and appended to the variable’s value. value is evaluated as an arithmetic expression and added to the variable’s current value. Positional parameters may not be assigned to with assignment statements. If IFS is null. All value s undergo tilde expansion. readonly. where c is the first character of the value of the IFS variable.. starting from one.4.".7 [Arrays]. other than the single digit 0. Positional parameters are assigned from the shell’s arguments when it is invoked.. starting from one. it expands to a single word with the value of each parameter separated by the first character of the IFS special variable.5. * Expands to the positional parameters. and quote removal (detailed below). page 14).. The set and shift builtins are used to set and unset them (see Chapter 4 [Shell Builtin Commands]. Expands to the positional parameters. and new values are appended to the array beginning at one greater than the array’s maximum index (for indexed arrays). Filename expansion is not performed. each parameter expands to a separate word. export. That is. page 35)...
3. the variable’s value is not unset (as it is when using ‘=’). "$*" is equivalent to "$1c$2c. When the expansion occurs within double quotes. typeset. That is.5 [Arithmetic Expansion]. If the double-quoted expansion occurs within a word.4. When applied to a string-valued variable. When the expansion occurs within double quotes. "$@" is equivalent to "$1" "$2" . declare. Assignment statements may also appear as arguments to the alias. If IFS is unset. it must be enclosed in braces.3 [Shell Functions].

) Expands to the current option flags as specified upon invocation. set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. In a () subshell.1 [Invoking Bash]. parameter. tilde expansion. # ? Expands to the number of positional parameters in decimal. by the set builtin command. This is set at shell initialization. Expands to the process id of the shell. expands to the last argument to the previous command. variable..) At shell startup.Chapter 3: Basic Shell Features
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part of the original word. (A hyphen.
. If Bash is invoked with a file of commands (see Section 3.8 [Shell Scripts]. "$@" and $@ expand to nothing (i. and arithmetic expansion and command substitution. not the subshell.5 Shell Expansions
Expansion is performed on the command line after it has been split into tokens. (An underscore. Expands to the exit status of the most recently executed foreground pipeline. Expands to the process id of the most recently executed background (asynchronous) command. and arithmetic expansion and command substitution (done in a left-to-right fashion). page 71).e. Subsequently. There are seven kinds of expansion performed: • brace expansion • tilde expansion • parameter and variable expansion • command substitution • arithmetic expansion • word splitting • filename expansion The order of expansions is: brace expansion. $0 is set to the name of that file. then $0 is set to the first argument after the string to be executed. This is performed at the same time as parameter. after expansion. if one is present. page 33). or those set by the shell itself (such as the ‘-i’ option). it is set to the filename used to invoke Bash. When there are no positional parameters. this parameter holds the name of the mail file. it expands to the process id of the invoking shell. Expands to the name of the shell or shell script. On systems that can support it. When checking mail. there is an additional expansion available: process substitution. and filename expansion. Otherwise. If Bash is started with the ‘-c’ option (see Section 6. as given by argument zero. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. variable. word splitting. they are removed).
$ ! 0
_
3. and the expansion of the last parameter is joined with the last part of the original word.

When either x or y begins with a zero. where x and y are either integers or single characters. Brace expansion is performed before any other expansions. Patterns to be brace expanded take the form of an optional preamble.’ may be quoted with a backslash to prevent its being considered part of a brace expression.incr]}.dist. page 25) is performed. followed by an optional postscript. word splitting.edit}.5. This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example: mkdir /usr/local/src/bash/{old. and the postscript is then appended to each resulting string.5..9 [Quote Removal].. and incr. the shell attempts to force all generated terms to contain the same number of digits.bugs} or chown root /usr/{ucb/{ex.new. When characters are supplied. page 16) and "${name[@]}" (see Section 6.b}e ade ace abe A sequence expression takes the form {x. Any incorrectly formed brace expansion is left unchanged. The results of each expanded string are not sorted. It is strictly textual. and filename expansion can change the number of words of the expansion. the string ‘${’ is not considered eligible for brace expansion. bash$ echo a{d. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. page 80). When the increment is supplied.c. To avoid conflicts with parameter expansion. After all expansions.4. Supplied integers may be prefixed with ‘0’ to force each term to have the same width.2 [Special Parameters]. Note that both x and y must be of the same type. To avoid conflicts with parameter expansion. The preamble is prefixed to each string contained within the braces. but the file names generated need not exist.1 Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be generated. A correctly-formed brace expansion must contain unquoted opening and closing braces. The default increment is 1 or -1 as appropriate. quote removal (see Section 3.lib/{ex?. the expression expands to each character lexicographically between x and y. it is used as the difference between each term. other expansions expand a single word to a single word. This mechanism is similar to filename expansion (see Section 3. followed by either a series of comma-separated strings or a seqeunce expression between a pair of braces. left to right order is preserved. A { or ‘.how_ex}}
. is an integer. and any characters special to other expansions are preserved in the result.?*. page 24). expanding left to right. Brace expansions may be nested.7 [Arrays].8 [Filename Expansion]. inclusive. the string ‘${’ is not considered eligible for brace expansion.y[. an optional increment. the expression expands to each number between x and y. When integers are supplied.5.
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Only brace expansion. zero-padding where necessary. For example. The only exceptions to this are the expansions of "$@" (see Section 3. and at least one unquoted comma or a valid sequence expression. inclusive.

The value of parameter is substituted. the value of the shell variable PWD replaces the tilde-prefix. as it would be displayed by the dirs builtin invoked with the characters following tilde in the tilde-prefix as an argument (see Section 6. If the login name is invalid.5. The braces are required when parameter is a positional parameter with more
. If the tilde-prefix. Each variable assignment is checked for unquoted tilde-prefixes immediately following a ‘:’ or the first ‘=’. and CDPATH. the value of the shell variable OLDPWD. The basic form of parameter expansion is ${parameter }. or parameter expansion. the word is left unchanged. and the shell assigns the expanded value. tilde expansion is also performed.8 [The Directory Stack]. If the tilde-prefix is ‘~-’. optionally prefixed by a ‘+’ or a ‘-’. If the tilde-prefix is ‘~+’. the tilde-prefix is replaced with the corresponding element from the directory stack. all of the characters up to the first unquoted slash (or all characters. The parameter name or symbol to be expanded may be enclosed in braces. In these cases. If this login name is the null string. the tilde is replaced with the value of the HOME shell variable.3 Shell Parameter Expansion
The ‘$’ character introduces parameter expansion. If HOME is unset. command substitution. Consequently.Chapter 3: Basic Shell Features
19
3. Otherwise. one may use file names with tildes in assignments to PATH. consists of a number without a leading ‘+’ or ‘-’. the characters in the tilde-prefix following the tilde are treated as a possible login name. and not within an embedded arithmetic expansion. or arithmetic expansion. is substituted. When braces are used. which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.5. MAILPATH. If none of the characters in the tilde-prefix are quoted. ‘+’ is assumed. or the tilde expansion fails.2 Tilde Expansion
If a word begins with an unquoted tilde character (‘~’). If the characters following the tilde in the tilde-prefix consist of a number N. the matching ending brace is the first ‘}’ not escaped by a backslash or within a quoted string. if there is no unquoted slash) are considered a tilde-prefix. sans the tilde. The following table shows how Bash treats unquoted tilde-prefixes: ~ ~/foo ~fred/foo The subdirectory foo of the home directory of the user fred ~+/foo ~-/foo ~N ~+N ~-N ‘$PWD/foo’ ‘${OLDPWD-’~-’}/foo’ The string that would be displayed by ‘dirs +N’ The string that would be displayed by ‘dirs +N’ The string that would be displayed by ‘dirs -N’ The value of $HOME ‘$HOME/foo’
3. the tilde-prefix is replaced with the home directory associated with the specified login name. page 81). command substitution. the home directory of the user executing the shell is substituted instead. if it is set.

If length is omitted. the expansion of word is assigned to parameter. command substitution. Otherwise. When not performing substring expansion. the value of parameter is substituted.5 [Shell Arithmetic]. If the first character of parameter is an exclamation point (!). the result is the length members of the array beginning with ${parameter[offset]}. Bash uses the value of the variable formed from the rest of parameter as the name of the variable. page 78). If parameter is ‘@’. Omitting the colon results in a test only for a parameter that is unset. Positional parameters and special parameters may not be assigned to in this way. the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell. Put another way. word is subject to tilde expansion. or when parameter is followed by a character that is not to be interpreted as part of its name. This is known as indirect expansion. if the colon is omitted. otherwise the expansion of word is substituted. exits. If parameter is an indexed array name subscripted by ‘@’ or ‘*’. This is referred to as Substring Expansion. length must evaluate to a number greater than or equal to zero. The value of parameter is then substituted. and arithmetic expansion. a level of variable indirection is introduced. if the colon is included. Otherwise. the operator tests for both parameter ’s existence and that its value is not null. ${parameter:−word} If parameter is unset or null. ${parameter:?word} If parameter is null or unset. the operator tests only for existence. this variable is then expanded and that value is used in the rest of the substitution. if it is not interactive. the value of parameter is substituted. nothing is substituted. rather than the value of parameter itself. using the form described below. The exclamation point must immediately follow the left brace in order to introduce indirection. parameter expansion. ${parameter:=word} If parameter is unset or null. the value is used as an offset from the end of the value of parameter. In each of the cases below.20
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than one digit. ${parameter:offset} ${parameter:offset:length} Expands to up to length characters of parameter starting at the character specified by offset. length and offset are arithmetic expressions (see Section 6. Bash tests for a parameter that is unset or null. the result is length positional parameters beginning at offset. The exceptions to this are the expansions of ${!prefix* } and ${!name [@]} described below. A negative offset is taken relative to one greater
. expands to the substring of parameter starting at the character specified by offset. If offset evaluates to a number less than zero. ${parameter:+word} If parameter is null or unset. the expansion of word is substituted.

Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the ‘:-’ expansion. If parameter is an array name subscripted by ‘*’ or ‘@’. If offset is 0. each key expands to a separate word. ${!prefix*} ${!prefix@} Expands to the names of variables whose names begin with prefix. ${!name[@]} ${!name[*]} If name is an array variable. then the result of the expansion is the value of parameter with the shortest matching pattern (the ‘%’ case) or the longest matching pattern (the ‘%%’ case) deleted. If the pattern matches a trailing portion of the expanded value of parameter. Substring indexing is zero-based unless the positional parameters are used. If parameter is an array variable subscripted with ‘@’ or ‘*’. the pattern removal
. ${parameter%word} ${parameter%%word} The word is expanded to produce a pattern just as in filename expansion.Chapter 3: Basic Shell Features
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than the maximum index of the specified array. expands to 0 if name is set and null otherwise. and the expansion is the resultant list. ${#parameter} The length in characters of the expanded value of parameter is substituted. the pattern removal operation is applied to each positional parameter in turn. each variable name expands to a separate word. If name is not an array. If parameter is ‘*’ or ‘@’. expands to the list of array indices (keys) assigned in name. separated by the first character of the IFS special variable. in which case the indexing starts at 1 by default. then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the ‘#’ case) or the longest matching pattern (the ‘##’ case) deleted. and the positional parameters are used. and the expansion is the resultant list. the value substituted is the number of elements in the array. When ‘@’ is used and the expansion appears within double quotes. page 24). If parameter is ‘@’ or ‘*’. Substring expansion applied to an associative array produces undefined results. When ‘@’ is used and the expansion appears within double quotes. If parameter is an array variable subscripted with ‘@’ or ‘*’. and the expansion is the resultant list. the pattern removal operation is applied to each positional parameter in turn.8 [Filename Expansion]. If the pattern matches the beginning of the expanded value of parameter. $@ is prefixed to the list. the value substituted is the number of positional parameters. the pattern removal operation is applied to each member of the array in turn.5. ${parameter#word} ${parameter##word} The word is expanded to produce a pattern just as in filename expansion (see Section 3. If parameter is ‘@’ or ‘*’.

’ operator converts matching uppercase letters to lowercase. Command substitution occurs when a command is enclosed as follows: $(command) or ‘command‘ Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command. If pattern begins with ‘#’. the case modification operation is applied to each member of the array in turn. the case modification operation is applied to each positional parameter in turn.. backslash retains its literal meaning except when followed by ‘$’. When the old-style backquote form of substitution is used. it must match at the end of the expanded value of parameter. and the expansion is the resultant list. If parameter is ‘@’ or ‘*’. If parameter is an array variable subscripted with ‘@’ or ‘*’. The command substitution $(cat file) can be replaced by the equivalent but faster $(< file). but they may be removed during word splitting.. and the expansion is the resultant list. it is treated like a ‘?’. which matches every character. the substitution operation is applied to each member of the array in turn. and the expansion is the resultant list. the ‘^’ and ‘.pattern} This expansion modifies the case of alphabetic characters in parameter.pattern} ${parameter. If parameter is ‘@’ or ‘*’. and the expansion is the resultant list. If pattern is omitted. The ‘^^’ and ‘. Normally only the first match is replaced. The first backquote not preceded by a
. If pattern begins with ‘%’. all matches of pattern are replaced with string. with any trailing newlines deleted. If pattern begins with ‘/’. or ‘\’. If parameter is an array variable subscripted with ‘@’ or ‘*’.
3. the substitution operation is applied to each positional parameter in turn. Parameter is expanded and the longest match of pattern against its value is replaced with string.5. ‘‘’. If string is null.22
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operation is applied to each member of the array in turn.4 Command Substitution
Command substitution allows the output of a command to replace the command itself.’ expansions convert each matched character in the expanded value. the ‘. it must match at the beginning of the expanded value of parameter. Embedded newlines are not deleted. The pattern is expanded to produce a pattern just as in filename expansion. matches of pattern are deleted and the / following pattern may be omitted. The ‘^’ operator converts lowercase letters matching pattern to uppercase. ${parameter^pattern} ${parameter^^pattern} ${parameter. ${parameter/pattern/string} The pattern is expanded to produce a pattern just as in filename expansion. and the expansion is the resultant list.’ expansions match and convert only the first character in the expanded value.

and <newline> at the beginning and end of the results of the previous expansions are ignored.5. word splitting and filename expansion are not performed on the results. It takes the form of <(list) or >(list) The process list is run with its input or output connected to a fifo or some file in ‘/dev/fd’. or its value is exactly <space><tab><newline>. When available. Note that no space may appear between the < or > and the left parenthesis.5. then sequences of the whitespace characters space and tab are ignored at the beginning and end of the word. command substitution.5. <tab>. If the expression is invalid. none are treated specially. command substitution. escape the inner backquotes with backslashes.
3.
3. process substitution is performed simultaneously with parameter and variable expansion. and arithmetic expansion. If IFS is unset. the file passed as an argument should be read to obtain the output of list. The format for arithmetic expansion is: $(( expression )) The expression is treated as if it were within double quotes. Any character in IFS that is not IFS
. the default. To nest when using the backquoted form. The shell treats each character of $IFS as a delimiter. and quote removal. If IFS has a value other than the default. and any sequence of IFS characters not at the beginning or end serves to delimit words. then sequences of <space>. If the >(list) form is used. all characters between the parentheses make up the command.7 Word Splitting
The shell scans the results of parameter expansion. command substitution. The name of this file is passed as an argument to the current command as the result of the expansion.5 Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. All tokens in the expression undergo parameter expansion. Bash prints a message indicating failure to the standard error and no substitution occurs. If the substitution appears within double quotes. The evaluation is performed according to the rules listed below (see Section 6. otherwise the construct would be interpreted as a redirection. as long as the whitespace character is in the value of IFS (an IFS whitespace character). Arithmetic expansions may be nested. and arithmetic expansion that did not occur within double quotes for word splitting. When using the $(command) form.
3. Command substitutions may be nested. If the <(list) form is used. page 78).6 Process Substitution
Process substitution is supported on systems that support named pipes (fifos) or the ‘/dev/fd’ method of naming open files.Chapter 3: Basic Shell Features
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backslash terminates the command substitution. writing to the file will provide input for list. but a double quote inside the parentheses is not treated specially. and splits the results of the other expansions into words on these characters.5 [Shell Arithmetic].

resulting from the expansion of parameters that have no values. and ‘*’ is used in a filename expansion context. no splitting is performed. The GLOBIGNORE shell variable may be used to restrict the set of filenames matching a pattern.1 [The Set Builtin]. the word is removed. the slash character must always be matched explicitly.’ will match.5. and the shell option nullglob is disabled. matches itself. The filenames ‘.1 Pattern Matching
Any character that appears in a pattern.3. See the description of shopt in Section 4.24
Bash Reference Manual
whitespace. If no matching file names are found. and dotglob options. However.
3.
3. other than the special pattern characters described below. The nul character may not occur in a pattern. along with any adjacent IFS whitespace characters. and no matches are found. unless the shell option dotglob is set. Note that if no expansion occurs. When a pattern is used for filename expansion.. including the null string. failglob. each matching filename that also matches one of the patterns in GLOBIGNORE is removed from the list of matches.2 [The Shopt Builtin].’ are always ignored when GLOBIGNORE is set and not null. If a parameter with no value is expanded within double quotes. the ‘. When the globstar shell option is enabled. A backslash escapes the following character. an error message is printed and the command is not executed. A sequence of IFS whitespace characters is also treated as a delimiter. then the word is regarded as a pattern. page 55.’ and ‘. In other cases. Matches any single character. the escaping backslash is discarded when matching. delimits a field.8. no word splitting occurs.3. If followed by a ‘/’.’.5. and replaced with an alphabetically sorted list of file names matching the pattern. make ‘. The dotglob option is disabled when GLOBIGNORE is unset.’ character is not treated specially. page 51). and ‘[’. two adjacent ‘*’s used as a single pattern will match all files and zero or more directories and subdirectories. Explicit null arguments ("" or ’’) are retained.
?
. nullglob. The special pattern characters must be quoted if they are to be matched literally. When matching a file name. the match is performed without regard to the case of alphabetic characters. If the failglob shell option is set. ‘?’.’ at the start of a filename or immediately following a slash must be matched explicitly. and no matches are found. so all other filenames beginning with a ‘. The special pattern characters have the following meanings: * Matches any string.8 Filename Expansion
After word splitting. If the nullglob option is set. Bash scans each word for the characters ‘*’. the character ‘. If one of these characters appears. If the shell option nocaseglob is enabled. If the value of IFS is null. To get the old behavior of ignoring filenames beginning with a ‘. Unquoted implicit null arguments. unless the ‘-f’ option has been set (see Section 4. If GLOBIGNORE is set. for a description of the nocaseglob. setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob shell option. a null argument results and is retained. the word is left unchanged.*’ one of the patterns in GLOBIGNORE. two adjacent ‘*’s will match only directories and subdirectories. are removed.

!(pattern-list) Matches anything except one of the given patterns. any character that sorts between those two characters.] matches the collating symbol symbol. in the default C locale. you can force the use of the C locale by setting the LC_COLLATE or LC_ALL environment variable to the value ‘C’. To obtain the traditional interpretation of ranges in bracket expressions.
If the extglob shell option is enabled using the shopt builtin. Composite patterns may be formed using one or more of the following sub-patterns: ?(pattern-list) Matches zero or one occurrence of the given patterns. ‘[a-dx-z]’ is equivalent to ‘[abcdxyz]’.9 Quote Removal
After the preceding expansions.symbol . inclusive. *(pattern-list) Matches zero or more occurrences of the given patterns.5. If the first character following the ‘[’ is a ‘!’ or a ‘^’ then any character not enclosed is matched. Within ‘[’ and ‘]’. Many locales sort characters in dictionary order. ‘’’. In the following description. and ‘"’ that did not result from one of the above expansions are removed. and the character ‘_’. +(pattern-list) Matches one or more occurrences of the given patterns. where class is one of the following classes defined in the posix standard: alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit A character class matches any character belonging to that class. an equivalence class can be specified using the syntax [=c =]. For example. and in these locales ‘[a-dx-z]’ is typically not equivalent to ‘[abcdxyz]’. A ‘−’ may be matched by including it as the first or last character in the set..]
Matches any one of the enclosed characters. A ‘]’ may be matched by including it as the first character in the set. several extended pattern matching operators are recognized. Within ‘[’ and ‘]’. Within ‘[’ and ‘]’. which matches all characters with the same collation weight (as defined by the current locale) as the character c. digits. @(pattern-list) Matches one of the given patterns.
3. is matched. it might be equivalent to ‘[aBbCcDdxXyYz]’.Chapter 3: Basic Shell Features
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[. character classes can be specified using the syntax [:class :]. using the current locale’s collating sequence and character set. the syntax [. for example. a pattern-list is a list of one or more patterns separated by a ‘|’. A pair of characters separated by a hyphen denotes a range expression. The word character class matches letters. if set. The sorting order of characters in range expressions is determined by the current locale and the value of the LC_COLLATE shell variable. all unquoted occurrences of the characters ‘\’.
..

from left to right.
. If the first character of the redirection operator is ‘>’. /dev/stdin File descriptor 0 is duplicated. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Bash handles several filenames specially when they are used in redirections. command substitution. If it expands to more than one word. tilde expansion. while the command ls 2>&1 > dirlist directs only the standard output to file dirlist. In the following descriptions. unless otherwise noted. the command ls > dirlist 2>&1 directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist.6 Redirections
Before a command is executed. /dev/stdout File descriptor 1 is duplicated. Bash attempts to open a TCP connection to the corresponding socket. In this case. and word splitting.is preceded by {varname }. for each redirection operator except >&. For example. If >&. its input and output may be redirected using a special notation interpreted by the shell. /dev/tcp/host/port If host is a valid hostname or Internet address. if the file descriptor number is omitted. filename expansion. Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname }. the redirection refers to the standard input (file descriptor 0). /dev/stderr File descriptor 2 is duplicated. arithmetic expansion. The word following the redirection operator in the following descriptions. Bash reports an error. the shell will allocate a file descriptor greater than 10 and assign it to {varname }. parameter expansion. the value of varname defines the file descriptor to close.26
Bash Reference Manual
3. because the standard error was made a copy of the standard output before the standard output was redirected to dirlist. quote removal. as described in the following table: /dev/fd/fd If fd is a valid integer. and port is an integer port number or service name. Redirections are processed in the order they appear. the redirection refers to the standard output (file descriptor 1). file descriptor fd is duplicated. Note that the order of redirections is significant.and <&-. is subjected to brace expansion.or <&. and the first character of the redirection operator is ‘<’.

The general format for appending output is: [n]>>word
3.
3. the first is preferred. A failure to open or create a file causes the redirection to fail. If the file does not exist it is created. the redirection is attempted even if the file named by word exists.Chapter 3: Basic Shell Features
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/dev/udp/host/port If host is a valid hostname or Internet address. or the redirection operator is ‘>’ and the noclobber option is not enabled. This is semantically equivalent to >word 2>&1
. If the file does not exist it is created. The general format for redirecting output is: [n]>[|]word If the redirection operator is ‘>’. Bash attempts to open a UDP connection to the corresponding socket.6. as they may conflict with file descriptors the shell uses internally.6.
3. If the redirection operator is ‘>|’. and port is an integer port number or service name. if it does exist it is truncated to zero size. or the standard input (file descriptor 0) if n is not specified. the redirection will fail if the file whose name results from the expansion of word exists and is a regular file. The general format for redirecting input is: [n]<word
3. and the noclobber option to the set builtin has been enabled.4 Redirecting Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word. Redirections using file descriptors greater than 9 should be used with care.2 Redirecting Output
Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n.3 Appending Redirected Output
Redirection of output in this fashion causes the file whose name results from the expansion of word to be opened for appending on file descriptor n.6. or the standard output (file descriptor 1) if n is not specified.6.1 Redirecting Input
Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n. There are two formats for redirecting standard output and standard error: &>word and >&word Of the two forms. or the standard output (file descriptor 1) if n is not specified.

The format for appending standard output and standard error is: &>>word This is semantically equivalent to >>word 2>&1
3. If the digits in word do not specify a file descriptor open
. a redirection error occurs.28
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3. If word is unquoted. file descriptor n is closed. If word evaluates to ‘-’. the format is: <<< word The word is expanded and supplied to the command on its standard input. In the latter case.6.6 Here Documents
This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. If any characters in word are quoted. This allows here-documents within shell scripts to be indented in a natural fashion. the file descriptor denoted by n is made to be a copy of that file descriptor.6. If word expands to one or more digits. and ‘‘’. If the digits in word do not specify a file descriptor open for input. The format of here-documents is: <<[−]word here-document delimiter No parameter expansion. the standard input (file descriptor 0) is used. all lines of the here-document are subjected to parameter expansion.6. If n is not specified.6.
3. command substitution. then all leading tab characters are stripped from input lines and the line containing delimiter. or filename expansion is performed on word. the character sequence \newline is ignored. the delimiter is the result of quote removal on word. command substitution.7 Here Strings
A variant of here documents. the standard output (file descriptor 1) is used. All of the lines read up to that point are then used as the standard input for a command. The operator [n]>&word is used similarly to duplicate output file descriptors. and ‘\’ must be used to quote the characters ‘\’.5 Appending Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be appended to the file whose name is the expansion of word. and arithmetic expansion. arithmetic expansion. If the redirection operator is ‘<<-’.
3. If n is not specified.8 Duplicating File Descriptors
The redirection operator [n]<&word is used to duplicate input file descriptors. ‘$’. and the lines in the here-document are not expanded.

but do not affect the current shell environment. command substitution. Otherwise. page 17). The text after the ‘=’ in each variable assignment undergoes tilde expansion. page 26). the redirection operator [n]>&digitmoves the file descriptor digit to file descriptor n. A redirection error causes the command to exit with a non-zero status. and redirections. the standard output and standard error are redirected as described previously. the variables are added to the environment of the executed command and do not affect the current shell environment. if n is omitted. assignments.5 [Shell Expansions].7 Executing Commands
3.
3. As a special case. If no command name results. the first word is taken to be the name of the command and the remaining words are the arguments. or on file descriptor 0 if n is not specified. parameter expansion. The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing. it is created. from left to right.
3.9 Moving File Descriptors
The redirection operator [n]<&digitmoves the file descriptor digit to file descriptor n. the variable assignments affect the current shell environment. 2. or the standard input (file descriptor 0) if n is not specified. Similarly. and word does not expand to one or more digits.Chapter 3: Basic Shell Features
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for output.10 Opening File Descriptors for Reading and Writing
The redirection operator [n]<>word causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n. If any of the assignments attempts to assign a value to a readonly variable. an error occurs. If the file does not exist.
3. 3. arithmetic expansion. Redirections are performed as described above (see Section 3.6. The words that are not variable assignments or redirections are expanded (see Section 3. and the command exits with a non-zero status.6. If no command name results. or the standard output (file descriptor 1) if n is not specified. and quote removal before being assigned to the variable. If any words remain after expansion.
. redirections are performed.1 Simple Command Expansion
When a simple command is executed. 4.7.6 [Redirections]. the shell performs the following expansions. 1. digit is closed after being duplicated to n. a redirection error occurs.

if it results in a simple command and an optional list of arguments. the shell searches for it in the list of shell builtins. Bash uses a hash table to remember the full pathnames of executable files to avoid multiple PATH searches (see the description of hash in Section 4. that builtin is invoked. 2. If there were no command substitutions. the command exits. which consists of the following: • open files inherited by the shell at invocation. that function is invoked as described in Section 3. the exit status of the command is the exit status of the last command substitution performed. and the file is not a directory. or popd.30
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If there is a command name left after expansion. it is invoked with the original command and the original command’s arguments as its arguments. the shell waits for the command to complete and collects its exit status. the shell attempts to locate it. 6. 4.
3. If the name does not match a function. and contains no slashes. Bash searches each element of $PATH for a directory containing an executable file by that name. the following actions are taken. If there exists a shell function by that name. it is assumed to be a shell script and the shell executes it as described in Section 3. If that function is not defined. execution proceeds as described below. Otherwise. 5. If the search is successful.3 Command Execution Environment
The shell has an execution environment. and the function’s exit status becomes the exit status of the shell. If the command was not begun asynchronously.7. page 33. Argument 0 is set to the name given. If one of the expansions contained a command substitution.1 [Bourne Shell Builtins]. the shell prints an error message and returns an exit status of 127. the command exits with a status of zero. and the remaining arguments to the command are set to the arguments supplied. If this execution fails because the file is not in executable format. A full search of the directories in $PATH is performed only if the command is not found in the hash table. the shell searches for a defined shell function named command_not_found_handle. or if the command name contains one or more slashes.8 [Shell Scripts]. as modified by redirections supplied to the exec builtin • the current working directory as set by cd. If the name is neither a shell function nor a builtin.7.
3. if any. or inherited by the shell at invocation • the file creation mode mask as set by umask or inherited from the shell’s parent • current traps set by trap • shell parameters that are set by variable assignment or with set or inherited from the shell’s parent in the environment
. If the command name contains no slashes. page 14. If that function exists. 1. If a match is found. the shell executes the named program in a separate execution environment.3 [Shell Functions]. If the search is unsuccessful. page 35). pushd. 3.2 Command Search and Execution
After a command has been split into words.

3. the value of $$. Bash clears the ‘-e’ option in such subshells. When not in posix mode. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. the default standard input for the command is the empty file ‘/dev/null’.4 [Environment]. page 31) • traps caught by the shell are reset to the values inherited from the shell’s parent. it is invoked in a separate execution environment that consists of the following. automatically marking it for export to child processes.7.2 [The Shopt Builtin]. Bash provides several ways to manipulate the environment. If the value of a parameter in the environment is modified. the values are inherited from the shell. • the shell’s open files. On invocation. This is a list of name-value pairs.2. Executed commands inherit the environment.7. including those of background jobs (see Section 3. If a command is followed by a ‘&’ and job control is not active. Changes made to the subshell environment cannot affect the shell’s execution environment.3 [Lists]. The export and ‘declare -x’ commands allow parameters and functions to be added to and deleted from the environment. plus any modifications and additions specified by redirections to the command • the current working directory • the file creation mode mask • shell variables and functions marked for export. Subshells spawned to execute command substitutions inherit the value of the ‘-e’ option from the parent shell. the new value becomes part of the environment. and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment. except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation.4 Environment
When a program is invoked it is given an array of strings called the environment. of the form name=value. passed in the environment (see Section 3. the invoked command inherits the file descriptors of the calling shell as modified by redirections. along with variables exported for the command. Unless otherwise noted. page 55) • shell aliases defined with alias (see Section 6. Command substitution. page 79) • various process ids.3.6 [Aliases]. the shell scans its own environment and creates a parameter for each name found. and traps ignored by the shell are ignored A command invoked in this separate environment cannot affect the shell’s execution environment. whose values may be
. The environment inherited by any executed command consists of the shell’s initial environment.Chapter 3: Basic Shell Features
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• shell functions defined during execution or inherited from the shell’s parent in the environment • options enabled at invocation (either by default or with command-line arguments) or by set • options enabled by shopt (see Section 4. and the value of $PPID When a simple command other than a builtin or shell function is to be executed. replacing the old. Otherwise. page 8). commands grouped with parentheses.

asynchronous commands ignore SIGINT and SIGQUIT in addition to these inherited handlers. then all parameter assignments are placed in the environment for a command. SIGTTOU. When Bash receives a SIGINT. in the absence of any traps. When job control is not in effect. the shell will use special values to indicate specific failure modes. and SIGTSTP. These assignment statements affect only the environment seen by that command. the exit status is greater than zero. not just those that precede the command name. If a command is not found. Bash ignores SIGTTIN. the child process created to execute it returns a status of 127.3. a command which exits with a zero exit status has succeeded. SIGTTOU.7. Under certain circumstances. When Bash invokes an external command.1 [The Set Builtin].2.6 Signals
When Bash is interactive.
3. All builtins return an exit status of 2 to indicate incorrect usage.3 [Lists].2. page 10) and some of the list constructs (see Section 3. and SIGINT is caught and handled (so that the wait builtin is interruptible). the variable ‘$_’ is set to the full path name of the command and passed to that command in its environment. so they may be used by the conditional and list constructs. If job control is in effect (see Chapter 7 [Job Control]. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. Bash uses the value 128+N as the exit status. The exit status is used by the Bash conditional commands (see Section 3. A non-zero exit status indicates failure. though. The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments. it breaks out of any executing loops.5 Exit Status
The exit status of an executed command is the value returned by the waitpid system call or equivalent function. All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure. plus any additions via the export and ‘declare -x’ commands. Commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN. For the shell’s purposes. If the ‘-k’ option is set (see Section 4. it ignores SIGTERM (so that ‘kill 0’ does not kill an interactive shell).4 [Shell Parameters]. Bash ignores SIGQUIT. as described in Section 3. When a command terminates on a fatal signal whose number is N.
3.4. the return status is 126. Non-builtin commands started by Bash have signal handlers set to the values inherited by the shell from its parent. less any pairs removed by the unset and ‘export -n’ commands.32
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modified in the shell.2 [Conditional Constructs]. page 8). the shell may use values above 125 specially. and SIGTSTP. If a command fails because of an error during expansion or redirection. In all cases. Exit statuses from shell builtins and compound commands are also limited to this range. Exit statuses fall between 0 and 255.7. If a command is found but is not executable. as explained below. page 15. page 51). page 89).
.

3.2 [The Shopt Builtin]. it should be removed from the jobs table with the disown builtin (see Section 7. the trap will not be executed until the command completes. the positional parameters are unset.Chapter 3: Basic Shell Features
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The shell exits by default upon receipt of a SIGHUP. or some other interpreter and write the rest of the script file in that language. executing filename arguments is equivalent to executing bash filename arguments if filename is an executable shell script.
. then exits. an interactive shell resends the SIGHUP to all jobs. When such a file is used as the first non-option argument when invoking Bash. In other words. the remainder of the line specifies an interpreter for the program. If Bash is waiting for a command to complete and receives a signal for which a trap has been set. and neither the ‘-c’ nor ‘-s’ option is supplied (see Section 6. The shell first searches for the file in the current directory. page 71).
3. Bash will perform this action on operating systems that do not handle it themselves. Stopped jobs are sent SIGCONT to ensure that they receive the SIGHUP. When Bash is waiting for an asynchronous command via the wait builtin. page 55). the reception of a signal for which a trap has been set will cause the wait builtin to return immediately with an exit status greater than 128. Perl. awk. it spawns a subshell to execute it. and looks in the directories in $PATH if not found there. If the huponexit shell option has been set with shopt (see Section 4. If the first line of a script begins with the two characters ‘#!’. Most versions of Unix make this a part of the operating system’s command execution mechanism. followed by the rest of the arguments. This mode of operation creates a non-interactive shell.8 Shell Scripts
A shell script is a text file containing shell commands.2 [Job Control Builtins]. with the exception that the locations of commands remembered by the parent (see the description of hash in Section 4. running or stopped. Bash reads and executes commands from the file. A shell script may be made executable by using the chmod command to turn on the execute bit. it sets the special parameter 0 to the name of the file. When Bash runs a shell script. Before exiting. so that the effect is as if a new shell had been invoked to interpret the script. Bash sends a SIGHUP to all jobs when an interactive login shell exits. immediately after which the trap is executed. Thus. The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file. and the positional parameters are set to the remaining arguments. page 35) are retained by the child. If no additional arguments are supplied. page 90) or marked to not receive SIGHUP using disown -h. if any are given. rather than the name of the shell. you can specify Bash. followed by the name of the script file.1 [Bourne Shell Builtins]. When Bash finds such a file while searching the $PATH for a command. This subshell reinitializes itself. To prevent the shell from sending the SIGHUP signal to a particular job. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters.1 [Invoking Bash].

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Bash scripts often begin with #! /bin/bash (assuming that Bash has been installed in ‘/bin’). since this ensures that Bash will be used to interpret the script. even if it is executed under another shell.

break break [n]
. logout.1 [Directory Stack Builtins]. continue. and the programmable completion facilities (see Section 8. page 90). : (a colon) : [arguments] Do nothing beyond expanding arguments and performing redirections.1 [Simple Commands].1 Bourne Shell Builtins
The following shell builtin commands are inherited from the Bourne Shell. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with ‘-’ as invalid options and require ‘--’ to prevent this interpretation. or zero if no commands are executed. and test builtins do not accept options and do not treat ‘--’ specially.Chapter 4: Shell Builtin Commands
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4 Shell Builtin Commands
Builtin commands are contained within the shell itself. the command history (see Section 9. page 117). each builtin command documented as accepting options preceded by ‘-’ accepts ‘--’ to signify the end of the options. (a period) . This builtin is equivalent to source. the PATH variable is used to find filename.8. false. When the name of a builtin command is used as the first word of a simple command (see Section 3.
4. Otherwise the positional parameters are unchanged.2. break. If any arguments are supplied. the shell executes the command directly. the directory stack (see Section 6. true.7 [Programmable Completion Builtins]. When Bash is not in posix mode. page 8). This section briefly describes the builtins which Bash inherits from the Bourne Shell.2 [Bash History Builtins]. The :. Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (see Section 7.2 [Job Control Builtins]. Builtin commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities. let. as well as the builtin commands which are unique to or have been extended in Bash. Unless otherwise noted. filename [arguments] Read and execute commands from the filename argument in the current shell context. page 81). they become the positional parameters when filename is executed. If filename does not contain a slash. or cannot be read. page 121). Many of the builtins have been extended by posix or Bash. the return status is non-zero. These commands are implemented as specified by the posix standard. without invoking another program. . The return status is zero. the current directory is searched if filename is not found in $PATH. and shift builtins accept and process arguments beginning with ‘-’ without requiring ‘--’. The return status is the exit status of the last command executed. The exit. If filename is not found.

the shell places a dash at the beginning of the zeroth argument passed to command. The return status is zero unless n is not greater than or equal to 1. it is used as a search path. redirections may be used to affect the current shell environment. If no command is specified. and the directory change is successful. If directory begins with a slash. If n is supplied. until. while. exit exit [n] Exit the shell. it replaces the shell without creating a new process. If directory is ‘-’. If there are no redirection errors. the exit status is that of the last command executed. until. If n is omitted. If the shell variable CDPATH exists. or select loop. export
. The ‘-c’ option causes command to be executed with an empty environment. and its exit status returned as the exit status of eval. exec exec [-cl] [-a name] [command [arguments]] If command is supplied. or if ‘-’ is the first argument. cd cd [-L|-P] [directory] Change the current working directory to directory. the shell passes name as the zeroth argument to command. symbolic links are followed by default or with the ‘-L’ option. If n is supplied. n must be greater than or equal to 1. the absolute pathname of the new working directory is written to the standard output. This is what the login program does. eval eval [arguments] The arguments are concatenated together into a single command. CDPATH is not used. or select loop. the nth enclosing loop is exited. If there are no arguments or only empty arguments. The ‘-P’ option means to not follow symbolic links. the execution of the nth enclosing loop is resumed. the return status is zero. otherwise the return status is non-zero. which is then read and executed. If a non-empty directory name from CDPATH is used. Any trap on EXIT is executed before the shell terminates. The return status is zero unless n is not greater than or equal to 1. while. it is equivalent to $OLDPWD. non-zero otherwise. the return status is zero. returning a status of n to the shell’s parent. If the ‘-l’ option is supplied.36
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Exit from a for. the value of the HOME shell variable is used. If directory is not given. n must be greater than or equal to 1. If ‘-a’ is supplied. continue continue [n] Resume the next iteration of an enclosing for. The return status is zero if the directory is successfully changed.

OPTIND is set to the index of the first non-option argument. but if more arguments are given in args. OPTIND is initialized to 1 each time the shell or a shell script is invoked. a list of exported names is displayed. it must be manually reset between multiple calls to getopts within the same shell invocation if a new set of parameters is to be used. If an invalid option is seen. In normal operation diagnostic messages are printed when invalid options or missing option arguments are encountered. otherwise the names refer to shell variables. even if the first character of optstring is not a colon. and the index of the next argument to be processed into the variable OPTIND. getopts places the next option in the shell variable name. or ‘-f’ is supplied with a name that is not a shell function. When an option requires an argument. When the end of options is encountered. silent error reporting is used. a question mark (‘?’) is placed in name. If the variable OPTERR is set to 0. and name is set to ‘?’. hash hash [-r] [-p filename] [-dt] [name]
. optstring contains the option characters to be recognized. the value of the variable is set to value. or if the ‘-p’ option is given. If a variable name is followed by =value. The colon (‘:’) and question mark (‘?’) may not be used as option characters. if not silent. Each time it is invoked. prints an error message and unsets OPTARG. getopts places that argument into the variable OPTARG. initializing name if it does not exist. If the first character of optstring is a colon. If getopts is silent. The ‘-p’ option displays output in a form that may be reused as input. no error messages will be displayed. The ‘-n’ option means to no longer mark each name for export. The return status is zero unless an invalid option is supplied. the name s refer to shell functions.Chapter 4: Shell Builtin Commands
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export [-fn] [-p] [name[=value]] Mark each name to be passed to child processes in the environment. getopts normally parses the positional parameters. getopts getopts optstring name [args] getopts is used by shell scripts to parse positional parameters. and getopts is not silent. if a character is followed by a colon. getopts can report errors in two ways. If a required argument is not found. If the ‘-f’ option is supplied. the option character found is placed in OPTARG and no diagnostic message is printed. one of the names is not a valid shell variable name. getopts parses those instead. getopts exits with a return value greater than zero. the option is expected to have an argument. which should be separated from it by white space. The shell does not reset OPTIND automatically. If getopts is silent. then a colon (‘:’) is placed in name and OPTARG is set to the option character found. OPTARG is unset. getopts places ‘?’ into name and. If no names are supplied. and a diagnostic message is printed.

If no arguments are given. returning either n or the exit status of the last command executed within the script as the exit status of the script. If a variable name is followed by =value. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied. This may also be used to terminate execution of a script being executed with the . Mark each name as readonly. The ‘-r’ option causes the shell to forget all remembered locations. return return [n] Cause a shell function to exit with the return value n. a list of all readonly names is printed. The ‘-l’ option causes output to be displayed in a format that may be reused as input. The return status is zero unless a name is not found or an invalid option is supplied. the return value is the exit status of the last command executed in the function. so they need not be searched for on subsequent invocations. The return status is zero unless an invalid option is supplied. Any command associated with the RETURN trap is executed before execution resumes after the function or script. If n is not supplied. or if only ‘-l’ is supplied. The ‘-d’ option causes the shell to forget the remembered location of each name. or the ‘-f’ option is supplied with a name that is not a shell function. The ‘-p’ option inhibits the path search. If multiple name arguments are supplied with ‘-t’ the name is printed before the hashed full pathname.. information about remembered commands is printed. If the ‘-t’ option is supplied. the ‘-A’ option means each name refers to an associative array variable. the pathname printed may contain symbolic links. the full pathname to which each name corresponds is printed. the pathname printed will not contain symbolic links. The values of these names may not be changed by subsequent assignment. each name refers to a shell function. If the ‘-L’ option is supplied. (or source) builtin.38
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Remember the full pathnames of commands specified as name arguments. If no name arguments are given. The ‘-p’ option causes output to be displayed in a format that may be reused as input. or if the ‘-p’ option is supplied. readonly readonly [-aApf] [name[=value]] .. the value of the variable is set to value. pwd pwd [-LP] Print the absolute pathname of the current working directory. The ‘-a’ option means each name refers to an indexed array variable. and filename is used as the location of name. or source. shift shift [n]
. one of the name arguments is not a valid shell variable or function name. The commands are found by searching through the directories listed in $PATH. If the ‘-P’ option is supplied. If the ‘-f’ option is supplied. The return status is non-zero if return is used outside a function and not during the execution of a script by .

The positional parameters from n+1 . 1 argument The expression is true if and only if the argument is not null. If the first argument is one of the unary conditional operators (see Section 6. The test and [ builtins evaluate conditional expressions using a set of rules based on the number of arguments. The return status is zero unless n is greater than $# or less than zero. 0 arguments The expression is false. .4 [Bash Conditional Expressions]. page 76). Returns the value of expr. $#-n. . the positional parameters are not changed. the last argument to the command must be a ]. expr1 -o expr2 True if either expr1 or expr2 is true. test does not accept any options. If the
. The ‘-a’ and ‘-o’ operators are considered binary operators when there are three arguments. page 76). test [ Evaluate a conditional expression expr. 3 arguments If the second argument is one of the binary conditional operators (see Section 6.4 [Bash Conditional Expressions]. Expressions may be combined using the following operators.
expr1 -a expr2 True if both expr1 and expr2 are true. 2 arguments If the first argument is ‘!’. see below. Expressions are composed of the primaries described below in Section 6. $# are renamed to $1 . page 76. the expression is true if the unary test is true. This may be used to override the normal precedence of operators. Each operator and operand must be a separate argument. it is assumed to be 1. the result of the expression is the result of the binary test using the first and third arguments as operands. If n is zero or greater than $#. non-zero otherwise. listed in decreasing order of precedence. ! expr ( expr ) True if expr is false. The evaluation depends on the number of arguments. nor does it accept and ignore an argument of ‘--’ as signifying the end of options.Chapter 4: Shell Builtin Commands
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Shift the positional parameters to the left by n.4 [Bash Conditional Expressions]. Parameters represented by the numbers $# to $#-n+1 are unset. When the [ form is used. the expression is false. If the first argument is not a valid unary operator. n must be a non-negative number less than or equal to $#. . . the expression is true if and only if the second argument is null. If n is not supplied.

4 arguments If the first argument is ‘!’. the shell displays the trap commands associated with each sigspec. the result is the one-argument test of the second argument. or only ‘-p’ is given.40
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first argument is ‘!’. the expression is parsed and evaluated according to precedence using the rules listed above. the command arg is executed each time a shell function or a script executed with the .2 [The Shopt Builtin]. every arithmetic for command. If a sigspec is ERR. the result is the negation of the threeargument expression composed of the remaining arguments. for command. trap prints the list of commands associated with each signal number in a form that may be reused as shell input. times times Print out the user and system times used by the shell and its children. select command. If arg is not present and ‘-p’ has been supplied. If arg is absent (and there is a single sigspec ) or equal to ‘-’. the command arg is executed whenever a simple command has a non-zero exit status. If a sigspec is DEBUG. If a sigspec is RETURN. then the signal specified by each sigspec is ignored by the shell and commands it invokes. Otherwise. Each sigspec is either a signal name or a signal number. The ERR trap is not executed if the failed command is part of the command list immediately following an until or while keyword. 5 or more arguments The expression is parsed and evaluated according to precedence using the rules listed above. If no arguments are supplied. case command. or source builtins finishes executing. If arg is the null string. If a sigspec is 0 or EXIT. page 55) for details of its effect on the DEBUG trap.] The commands in arg are to be read and executed when the shell receives signal sigspec.3. or if
.. If the first argument is exactly ‘(’ and the third argument is exactly ‘)’. part of a command executed in a && or || list. arg is executed when the shell exits. Refer to the description of the extdebug option to the shopt builtin (see Section 4. part of the test following the if or elif reserved words. The ‘-l’ option causes the shell to print a list of signal names and their corresponding numbers.. the command arg is executed before every simple command. subject to the following conditions. The return status is zero. and before the first command executes in a shell function. trap trap [-lp] [arg] [sigspec . the value is the negation of the two-argument test using the second and third arguments. each specified signal’s disposition is reset to the value it had when the shell was started. Signal names are case insensitive and the SIG prefix is optional. the expression is false. Otherwise.

the current value of the mask is printed. Signals ignored upon entry to the shell cannot be trapped or reset. the mask is printed in a symbolic format.. If no options are supplied. The return status is zero if the mode is successfully changed or if no mode argument is supplied. If the ‘-f’ option is given. alias alias [-p] [name[=value] . it is interpreted as an octal number. each name refers to a shell variable. If mode is omitted. Aliases are described in Section 6.6 [Aliases].
4. the name s refer to shell functions. Thus. the output is in a form that may be reused as input. an alias is defined for each name whose value is given. or the ‘-v’ option is given. alias prints the list of aliases on the standard output in a form that allows them to be reused as input.Chapter 4: Shell Builtin Commands
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the command’s return status is being inverted using !. and non-zero otherwise. Some of these commands are specified in the posix standard. These are the same conditions obeyed by the errexit option.] Without arguments or with the ‘-p’ option. The return status is zero unless a name is readonly. and mode is omitted. the name and value of the alias is printed. If the ‘-S’ option is supplied without a mode argument.2 Bash Builtin Commands
This section describes builtin commands which are unique to or have been extended in Bash. If arguments are supplied. The return status is zero unless a sigspec does not specify a valid signal. each number of the umask is subtracted from 7. a umask of 022 results in permissions of 755. and the function definition is removed. umask umask [-p] [-S] [mode] Set the shell process’s file creation mask to mode.. unset unset [-fv] [name] Each variable or function name is removed. if not. page 79. Readonly variables and functions may not be unset. If no value is given. Trapped signals that are not being ignored are reset to their original values in a subshell or subshell environment when one is created. Note that when the mode is interpreted as an octal number. If the ‘-p’ option is supplied. bind bind bind bind bind [-m [-m [-m [-m keymap] keymap] keymap] keymap] [-lpsvPSV] [-q function] [-u function] [-r keyseq] -f filename -x keyseq:shell-command
. If mode begins with a digit. it is interpreted as a symbolic mode mask similar to that accepted by the chmod command.

-u function Unbind all keys bound to the named function. if supplied.3 [Readline Init File]. or set a Readline variable.
-S
-f filename Read key bindings from filename. page 96). the shell sets the READLINE_ LINE variable to the contents of the Readline line buffer and the READLINE_POINT variable to the current location of the insertion point. but each binding or command must be passed as a separate argument. have the following meanings: -m keymap Use keymap as the keymap to be affected by the subsequent bindings. If the executed command changes the value of READLINE_ LINE or READLINE_POINT. -q function Query about which keys invoke the named function. -r keyseq Remove any current binding for keyseq. Acceptable keymap names are emacs. -l -p -P -v -V -s List the names of all Readline functions. Display Readline function names and bindings in such a way that they can be used as input or in a Readline initialization file. vi-move. Display Readline variable names and values in such a way that they can be used as input or in a Readline initialization file. vi-command.
. emacs-meta. emacs-standard.42
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bind [-m keymap] keyseq:function-name bind readline-command Display current Readline (see Chapter 8 [Command Line Editing]. emacs-ctlx. List current Readline function names and bindings. Display Readline key sequences bound to macros and the strings they output in such a way that they can be used as input or in a Readline initialization file. and vi-insert. vi is equivalent to vi-command.g. emacs is equivalent to emacs-standard. vi. List current Readline variable names and values. Display Readline key sequences bound to macros and the strings they output. Options. bind a key sequence to a Readline function or macro. Each non-option argument is a command as it would appear in a Readline initialization file (see Section 8. -x keyseq:shell-command Cause shell-command to be executed whenever keyseq is entered. ‘"\C-x\C-r":re-read-init-file’. When shell-command is executed. those new values will be reflected in the editing state. page 93) key and function bindings. e..

The ‘-p’ option means to use a default value for PATH that is guaranteed to find all of the standard utilities. declare will display the attributes and values of all variables having the attributes specified by the additional options.. or source builtins). If there is a shell function named ls. the return status is zero if command is found. If a non-negative integer is supplied as expr. subroutine name. When ‘-p’ is supplied without name arguments. declare will display
. The ‘-v’ option causes a single word indicating the command or file name used to invoke command to be displayed. passing it args. Without expr. In this case. caller displays the line number and source filename of the current subroutine call. command command [-pVv] command [arguments . Only shell builtin commands or commands found by searching the PATH are executed. The return value is 0 unless the shell is not executing a subroutine call or expr does not correspond to a valid position in the call stack.. builtin builtin [shell-builtin [args]] Run a shell builtin. If no name s are given. to print a stack trace. and the exit status of command otherwise. and non-zero if not. caller displays the line number. the ‘-V’ option produces a more verbose description. a description of command is printed. running ‘command ls’ within the function will execute the external command ls instead of calling the function recursively. additional options are ignored. The current frame is frame 0.] Runs command with arguments ignoring any shell function named command. then display the values of variables instead. for example. The ‘-p’ option will display the attributes and values of each name. If no other options are supplied with ‘-p’. The return status is nonzero if shell-builtin is not a shell builtin command.] Declare variables and give them attributes. caller caller [expr] Returns the context of any active subroutine call (a shell function or a script executed with the . If either the ‘-V’ or ‘-v’ option is supplied.Chapter 4: Shell Builtin Commands
43
The return status is zero unless an invalid option is supplied or an error occurs. This is useful when defining a shell function with the same name as a shell builtin. retaining the functionality of the builtin within the function. and return its exit status. When ‘-p’ is used with name arguments. declare declare [-aAfFilrtux] [-p] [name[=value] .. and source file corresponding to that position in the current execution call stack. This extra information may be used. The return status in this case is 127 if command cannot be found or an error occurred..

-l -r -t
-u -x
Using ‘+’ instead of ‘-’ turns off the attribute instead. page 80).5 [Shell Arithmetic]. When the variable is assigned a value. These names cannot then be assigned values by subsequent assignment statements or unset. Traced functions inherit the DEBUG and RETURN traps from the calling shell. an attempt is made to define a function using ‘-f foo=bar’. The variable is to be treated as an integer. page 55). one of the names is not a valid shell variable name. with the exceptions that ‘+a’ may not be used to destroy an array variable and ‘+r’ will not remove the readonly attribute. When used in a function. as with the local command. The return status is zero unless an invalid option is encountered. the value of the variable is set to value. an attempt is made to turn off array status for an array variable. page 78) is performed when the variable is assigned a value. The lower-case attribute is disabled. or an attempt is made to display a non-existent function with ‘-f’.44
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the attributes and values of all shell variables. declare makes each name local.3. Use function names only.7 [Arrays].2 [The Shopt Builtin]. all upper-case characters are converted to lower-case. ‘-F’ implies ‘-f’. arithmetic evaluation (see Section 6. page 80). If a variable name is followed by =value. The ‘-F’ option inhibits the display of function definitions. Give each name the trace attribute. If the extdebug shell option is enabled using shopt (see Section 4. only the function name and attributes are printed. page 80). The trace attribute has no special meaning for variables. all lower-case characters are converted to upper-case.7 [Arrays]. The upper-case attribute is disabled. the source file name and line number where the function is defined are displayed as well. The ‘-f’ option will restrict the display to shell functions. Each name is an associative array variable (see Section 6. an attempt is made to turn off readonly status for a readonly variable. When the variable is assigned a value. echo
. Make name s readonly. Mark each name for export to subsequent commands via the environment. an attempt is made to assign a value to a readonly variable.7 [Arrays]. an attempt is made to assign a value to an array variable without using the compound assignment syntax (see Section 6. The following options can be used to restrict output to variables with the specified attributes or to give variables attributes: -a -A -f -i Each name is an indexed array variable (see Section 6.

interpretation of the following backslash-escaped characters is enabled.. The ‘-d’ option will delete a builtin loaded with ‘-f’. type ‘enable -n test’. For example.] Output the arg s. echo does not interpret ‘--’ to mean the end of options. or no name arguments appear. The ‘-f’ option means to load the new builtin command name from shared object filename. even though the shell normally searches for builtins before disk commands. The ‘-a’ option means to list each builtin with an indication of whether or not it is enabled.. to use the test binary found via $PATH instead of the shell builtin version. the name s become disabled. echo interprets the following escape sequences: \a \b \c \e \f \n \r \t \v \\ \0nnn \xHH enable enable [-a] [-dnps] [-f filename] [name . If the ‘-p’ option is supplied. the list consists of all enabled shell builtins. Otherwise name s are enabled.. the trailing newline is suppressed. even on systems where they are interpreted by default. The return status is always 0. terminated with a newline. With no other arguments. separated by spaces. If ‘-n’ is specified.Chapter 4: Shell Builtin Commands
45
echo [-neE] [arg . The ‘-E’ option disables the interpretation of these escape characters. If ‘-n’ is used. Disabling a builtin allows a disk command which has the same name as a shell builtin to be executed without specifying a full pathname. on systems that support dynamic loading. a list of shell builtins is printed. If the ‘-e’ option is given. The xpg_ echo shell option may be used to dynamically determine whether or not echo expands these escape characters by default.] Enable and disable builtin shell commands. alert (bell) backspace suppress further output escape form feed new line carriage return horizontal tab vertical tab backslash the eight-bit character whose value is the octal value nnn (zero to three octal digits) the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
..

and assigned value. an invalid name is supplied. otherwise a list of the builtins is printed.
. have the following meanings: -n -O Copy at most count lines. local local [option] name[=value] .. If pattern is specified. The option can be any of the options accepted by declare. if supplied. or name is a readonly variable. Options. if supplied. help help [-dms] [pattern] Display helpful information about builtin commands. all lines are copied. If ‘-s’ is used with ‘-f’. mapfile mapfile [-n count] [-O origin] [-s count] [-t] [-u fd] [ -C callback] [-c quantum] [array] Read lines from the standard input into the indexed array variable array. the new builtin becomes a special builtin (see Section 4. local can only be used within a function. If the last expression evaluates to 0. let let expression [expression] The let builtin allows arithmetic to be performed on shell variables. have the following meanings: -d -m -s Display a short description of each pattern Display the description of each pattern in a manpage-like format Display only a short usage synopsis for each pattern
The return status is zero unless no command matches pattern. returning a status of n to the shell’s parent. a list of the shell builtins is displayed. The return status is zero unless local is used outside a function. The return status is zero unless a name is not a shell builtin or there is an error loading a new builtin from a shared object. it makes the variable name have a visible scope restricted to that function and its children. Each expression is evaluated according to the rules given below in Section 6. otherwise 0 is returned. or from file descriptor fd if the ‘-u’ option is supplied. help gives detailed help on all commands matching pattern. a local variable named name is created. page 78. For each argument.. The ‘-s’ option restricts enable to the posix special builtins. The default index is 0.4 [Special Builtins]. Begin assigning to array at index origin. Options.5 [Shell Arithmetic]. let returns 1. page 59). The variable MAPFILE is the default array.46
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If there are no options. logout logout [n] Exit a login shell. If count is 0.

The ‘-v’ option causes the output to be assigned to the variable var rather than being printed to the standard output.] One line is read from the standard input. and so on. printf printf [-v var] format [arguments] Write the formatted arguments to the standard output under the control of the format. which are simply copied to standard output. The return value is zero on success. and the first word is assigned to the first name. Evaluate callback each time quantumP lines are read. Specify the number of lines read between each call to callback. or array is not an indexed array. and ‘%q’ causes printf to output the corresponding argument in a format that can be reused as shell input. the second word to the second name. backslashes in ‘\’’. and format specifications. Read lines from file descriptor fd instead of the standard input. which are converted and copied to the standard output. ‘\"’. had been supplied. If there are fewer words read from the input stream than names. When callback is evaluated. and octal escapes beginning with ‘\0’ may contain up to four digits). The format is a character string which contains three types of objects: plain characters. read read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [p prompt] [-t timeout] [-u fd] [name . with leftover words and their intervening separators assigned to the last name. (except that ‘\c’ terminates output. or from the file descriptor fd supplied as an argument to the ‘-u’ option. The characters in the value of the IFS variable are used to split
. character escape sequences. each of which causes printing of the next successive argument. mapfile returns successfully unless an invalid option or option argument is supplied. If not supplied with an explicit origin. array is invalid or unassignable. the default quantum is 5000. non-zero on failure. mapfile will clear array before assigning to it. The ‘-c’ option specifies quantum.. In addition to the standard printf(1) formats. it is supplied the index of the next array element to be assigned as an additional argument. callback is evaluated after the line is read but before the array element is assigned. the remaining names are assigned empty values. the extra format specifications behave as if a zero value or null string. ‘%b’ causes printf to expand backslash escape sequences in the corresponding argument.Chapter 4: Shell Builtin Commands
47
-s -t -u -C -c
Discard the first count lines read. The format is reused as necessary to consume all of the arguments. as appropriate. If the format requires more arguments than are supplied. and ‘\?’ are not removed. Remove a trailing newline from each line read.
If ‘-C’ is specified without ‘-c’..

the line read is assigned to the variable REPLY. If Readline is being used to read the line. but honor a delimiter if fewer than nchars characters are read before the delimiter. -p prompt Display prompt. -N nchars read returns after reading exactly nchars characters rather than waiting for a complete line of input. read returns success if input is available on the specified file descriptor.
-d delim -e
-i text
-n nchars read returns after reading nchars characters rather than waiting for a complete line of input. if line editing was not previously active) editing settings. unless EOF is encountered or read times out. a backslash-newline pair may not be used as a line continuation. -r If this option is given. page 93) is used to obtain the line. or an invalid file descriptor is supplied as the argument to ‘-u’. In particular. Options. Delimiter characters encountered in the input are not treated specially and do not cause read to return until nchars characters are read. The return code is zero. Cause read to time out and return failure if a complete line of input is not read within timeout seconds. timeout may be a decimal number with a fractional portion following the decimal point. unless end-of-file is encountered. it has no effect when reading from regular files. backslash does not act as an escape character. Silent mode.48
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the line into words. The prompt is displayed only if input is coming from a terminal. rather than newline. if supplied.
-s -t timeout
. starting at 0. If timeout is 0. All elements are removed from aname before the assignment. before attempting to read any input. If input is coming from a terminal. The backslash character ‘\’ may be used to remove any special meaning for the next character read and for line continuation. The first character of delim is used to terminate the input line. Other name arguments are ignored. The backslash is considered to be part of the line. Readline (see Chapter 8 [Command Line Editing]. Readline uses the current (or default. read times out (in which case the return code is greater than 128). text is placed into the editing buffer before editing begins. If no names are supplied. have the following meanings: -a aname The words are assigned to sequential indices of the array variable aname. This option is only effective if read is reading input from a terminal. pipe. without a trailing newline. failure otherwise. characters are not echoed. or other special file. The exit status is greater than 128 if the timeout is exceeded.

type either returns the name of the disk file that would be executed. then nothing is printed. shell function. ulimit ulimit [-abcdefilmnpqrstuvxHST] [limit] ulimit provides control over the resources available to processes started by the shell. ‘-p’ and ‘-P’ print the hashed value. If the ‘-p’ option is used. type type [-afptP] [name ... and type returns a failure status. shell builtin.] For each name. The ‘-P’ option forces a path search for each name. If the ‘-a’ option is used.. not necessarily the file that appears first in $PATH.Chapter 4: Shell Builtin Commands
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-u fd readarray
Read input from file descriptor fd. ‘builtin’. or from file descriptor fd if the ‘-u’ option is supplied.] The typeset command is supplied for compatibility with the Korn shell.. type does not attempt to find shell functions. ‘function’. it is interpreted as follows: -S Change and report the soft limit associated with a resource. If an option is given. The return status is zero if all of the names are found. respectively. If the ‘-t’ option is used. disk file. source source filename A synonym for . If the ‘-f’ option is used. type returns all of the places that contain an executable named file. non-zero if any are not found. if and only if the ‘-p’ option is not also used.
readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [ -C callback] [-c quantum] [array] Read lines from the standard input into the indexed array variable array. ‘file’ or ‘keyword’. even if ‘-t’ would not return ‘file’. indicate how it would be interpreted if used as a command name. it has been deprecated in favor of the declare builtin command. or shell reserved word. If a command is hashed. as with the command builtin. This includes aliases and functions. A synonym for mapfile. on systems that allow such control. typeset typeset [-afFrxi] [-p] [name[=value] . page 35). If the name is not found. however. (see Section 4. or nothing if ‘-t’ would not return ‘file’.1 [Bourne Shell Builtins]. if name is an alias. type prints a single word which is one of ‘alias’.
.

The maximum socket buffer size. the special limit values hard. The maximum real-time scheduling priority. a soft limit may be increased up to the value of the hard limit. the current soft limit. ] Remove each name from the list of aliases. soft. The maximum number of file locks.. The maximum number of bytes in POSIX message queues. if neither ‘-H’ nor ‘-S’ is supplied. unalias unalias [-a] [name . The maximum size of core files created. The maximum number of open file descriptors (most systems do not allow this value to be set). The maximum resident set size (many systems do not honor this limit). Aliases are described in Section 6. and unlimited stand for the current hard limit. which is in units of 512-byte blocks. The maximum size of files written by the shell and its children. A hard limit cannot be increased by a non-root user once it is set. The maximum amount of cpu time in seconds. which is in seconds. which are unscaled values. If ‘-a’ is supplied. If no option is given. Values are in 1024-byte increments. The maximum scheduling priority ("nice"). the current value of the soft limit for the specified resource is printed.. ‘-p’. The maximum stack size. The maximum amount of virtual memory available to the process. The maximum size that may be locked into memory. all aliases are removed. and no limit. page 79.
If limit is given. All current limits are reported. The maximum size of a process’s data segment. except for ‘-t’. and ‘-n’ and ‘-u’. both the hard and soft limits are set. unless the ‘-H’ option is supplied. it is the new value of the specified resource. Otherwise. The maximum number of processes available to a single user.50
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-H -a -b -c -d -e -f -i -l -m -n -p -q -r -s -t -u -v -x -T
Change and report the hard limit associated with a resource. When setting new limits.6 [Aliases]. or an error occurs while setting a new limit. respectively. The maximum number of pending signals. then ‘-f’ is assumed. The pipe buffer size.
. The maximum number of threads. The return status is zero unless an invalid option or argument is supplied.

set set [--abefhkmnptuvxBCEHPT] [-o option] [argument . set allows you to change the values of shell options and set the positional parameters.1 [Simple Commands]. This option is enabled by default. sorted according to the current locale.] If no options or arguments are supplied. page 13) returns a non-zero status.3 Modifying Shell Behavior
4. they set or unset shell attributes. rather than before printing the next primary prompt. part of any command executed in a && or || list except the command following the final && or ||.2. in a format that may be reused as input for setting or resetting the currently-set variables. set displays the names and values of all shell variables and functions. When options are supplied.3 [Command Execution Environment]. or one of the commands executed as part of a command list enclosed by braces (see Section 3. only shell variables are listed..4. page 30).2 [Pipelines].. This option applies to the shell environment and each subshell environment separately (see Section 3. A trap on ERR. Exit immediately if a pipeline (see Section 3. have the following meanings: -a -b -e Mark variables and function which are modified or created for export to the environment of subsequent commands. page 13). Read-only variables cannot be reset. part of the test in an if statement.
.7.] set [+abefhkmnptuvxBCEHPT] [+o option] [argument . Options.. if specified.3.3 [Command Grouping]. and may cause subshells to exit before executing all the commands in the subshell.2.Chapter 4: Shell Builtin Commands
51
4. any command in a pipeline but the last.1 The Set Builtin
This builtin is so complicated that it deserves its own section.2.3 [Command Grouping]. page 8). The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword. In posix mode. page 8)..2. -f -h Disable filename expansion (globbing). Cause the status of terminated background jobs to be reported immediately. or if the command’s return status is being inverted with !. a subshell command enclosed in parentheses (see Section 3. is executed before the shell exits. if set. which may consist of a single simple command (see Section 3.4. or to display the names and values of shell variables. Locate and remember (hash) commands as they are looked up for execution.

Same as -b. This option is on by default in interactive shells. braceexpand Same as -B. noexec noglob nolog notify nounset Same as -n. keyword monitor noclobber Same as -C. Currently ignored. Read commands but do not execute them. not just those that precede the command name.
errexit errtrace functrace
ignoreeof
.
-m -n
-o option-name Set the option corresponding to option-name : allexport Same as -a. Job control is enabled (see Chapter 7 [Job Control]. page 89). Same as -T. hashall histexpand Same as -H.52
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-k
All arguments in the form of assignment statements are placed in the environment for a command. Same as -u. Same as -f. This also affects the editing interface used for read -e. Same as -e. history Enable command history. This option is ignored by interactive shells. Same as -k. this may be used to check a script for syntax errors. page 93). emacs Use an emacs-style line editing interface (see Chapter 8 [Command Line Editing]. page 121. Same as -h. Same as -E. Same as -m.1 [Bash History Facilities]. An interactive shell will not exit upon reading EOF. as described in Section 9.

shell functions are not inherited from the environment. CDPATH and GLOBIGNORE variables. and the SHELLOPTS. Same as -P. Print a trace of simple commands. An error message will be written to the standard error. This is intended to make Bash behave as a strict superset of that standard. Exit after reading and executing one command. The shell will perform brace expansion (see Section 3. In this mode.1 [Brace Expansion]. This option is on by default. the effective user id is not reset. Use a vi-style line editing interface. for commands.Chapter 4: Shell Builtin Commands
53
onecmd physical pipefail
Same as -t. if they appear in the environment. case commands.5.11 [Bash POSIX Mode]. The value of the PS4 variable is expanded and the resultant value is printed before the command and its expanded arguments. This also affects the editing interface used for read -e.
Turn on privileged mode. and a noninteractive shell will exit. This option is disabled by default.
-t -u
-v -x
-B
. Print shell input lines as they are read. Same as -p. page 84). page 18). and the -p option is not supplied. are ignored.
posix
privileged verbose vi xtrace -p Same as -v. Turning this option off causes the effective user and group ids to be set to the real user and group ids. Treat unset variables and parameters other than the special parameters ‘@’ or ‘*’ as an error when performing parameter expansion. select commands. If the -p option is supplied at startup. If set. and arithmetic for commands and their arguments or associated word lists after they are expanded and before they are executed. the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status. the $BASH_ENV and $ENV files are not processed. Change the behavior of Bash where the default operation differs from the posix standard to match the standard (see Section 6. BASHOPTS. If the shell is started with the effective user (group) id not equal to the real user (group) id. these actions are taken and the effective user id is set to the real user id. or zero if all commands in the pipeline exit successfully. Same as -x.

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-C -E
Prevent output redirection using ‘>’. The remaining N arguments are positional parameters and are assigned. If there are no arguments. If set. and commands executed in a subshell environment.
--
-
Using ‘+’ rather than ‘-’ causes these options to be turned off. Otherwise. page 123). If no arguments follow this option. pwd /usr If set -P is on. cause all remaining arguments to be assigned to the positional parameters. The return status is always zero unless an invalid option is supplied. in order. echo $PWD /usr/sys $ cd .
. any trap on ERR is inherited by shell functions. if ‘/usr/sys’ is a symbolic link to ‘/usr/local/sys’ then: $ cd /usr/sys.. Signal the end of options. any trap on DEBUG and RETURN are inherited by shell functions. Enable ‘!’ style history substitution (see Section 9. . . The ‘-x’ and ‘-v’ options are turned off. The physical directory is used instead. echo $PWD /usr/local/sys $ cd . $2. The DEBUG and RETURN traps are normally not inherited in such cases. The options can also be used upon invocation of the shell. then the positional parameters are unset.. do not follow symbolic links when performing commands such as cd which change the current directory. Bash follows the logical chain of directories when performing commands which change the current directory.3 [History Interaction]. even if some of them begin with a ‘-’. $N. command substitutions.. the positional parameters remain unchanged. ‘>&’. then: $ cd /usr/sys. to $1. . For example. and ‘<>’ from overwriting existing files. the positional parameters are set to the arguments. and commands executed in a subshell environment. The ERR trap is normally not inherited in such cases. The current set of options may be found in $-. This option is on by default for interactive shells. pwd /usr/local
-H
-P
-T
If set. If set. The special parameter # is set to N. By default. command substitutions..

or with the ‘-p’ option. the corrected path is printed. the shopt options are disabled (off) by default.
checkhash
. and a character too many..
cdable_vars If this is set. The list of shopt options is: autocd If set.2 The Shopt Builtin
This builtin allows you to change additional shell optional behavior. If a hashed command no longer exists. If this is set. Suppresses normal output.. Disable (unset) each optname.3. an argument to the cd builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to. The errors checked for are transposed characters.1 [The Set Builtin]. respectively. a missing character. page 51). If multiple optname arguments are given with ‘-q’.Chapter 4: Shell Builtin Commands
55
4. a command name that is the name of a directory is executed as if it were the argument to the cd command. If a correction is found. cdspell If set. minor errors in the spelling of a directory component in a cd command will be corrected. Restricts the values of optname to be those defined for the ‘-o’ option to the set builtin (see Section 4. With no options. This option is only used by interactive shells. The ‘-p’ option causes output to be displayed in a form that may be reused as input.
-o
If either ‘-s’ or ‘-u’ is used with no optname arguments. the return status indicates whether the optname is set or unset. Bash checks that a command found in the hash table exists before trying to execute it. the return status is zero unless an optname is not a valid shell option. the return status is zero if all optnames are enabled. shopt shopt [-pqsu] [-o] [optname . a list of all settable options is displayed. Other options have the following meanings: -s -u -q Enable (set) each optname.3. the display is limited to those options which are set or unset. nonzero otherwise. This option is only used by interactive shells. a normal path search is performed. with an indication of whether or not each is set. and the command proceeds. Unless otherwise noted. non-zero otherwise. When setting or unsetting options. The return status when listing options is zero if all optnames are enabled.] Toggle the values of variables controlling optional shell behavior.

If set. 3. Bash includes filenames beginning with a ‘. and the shell is executing in a subroutine (a shell function or a shell script executed by the . this causes the exit to be deferred until a second exit is attempted without an intervening command (see Chapter 7 [Job Control].2 [Bash Variables]. and subshells invoked with ( command ) inherit the DEBUG and RETURN traps. 5.6 [Aliases]. Function tracing is enabled: command substitution. Bash changes its behavior to that of version 3. This option is enabled by default for interactive shells. shell functions. Bash checks the window size after each command and. If set. Bash attempts spelling correction on directory names during word completion if the directory name initially supplied does not exist. If the command run by the DEBUG trap returns a value of 2. page 41) displays the source file name and line number corresponding to each function name supplied as an argument. cmdhist If set. 4. If the command run by the DEBUG trap returns a non-zero value.2 [Bash Builtins]. page 61). The shell always postpones exiting if any jobs are stopped. Bash attempts to save all lines of a multiple-line command in the same history entry.
compat31 dirspell
dotglob execfail
expand_aliases If set. This allows easy re-editing of multi-line commands. If any jobs are running. or source builtins). BASH_ARGC and BASH_ARGV are updated as described in their descriptions (see Section 5. 2. updates the values of LINES and COLUMNS. If this is set. Section 6. a non-interactive shell will not exit if it cannot execute the file specified as an argument to the exec builtin command. a call to return is simulated. page 89).
.’ in the results of filename expansion. aliases are expanded as described below under Aliases. checkwinsize If set. behavior intended for use by debuggers is enabled: 1. Bash lists the status of any stopped and running jobs before exiting an interactive shell. if necessary. An interactive shell does not exit if exec fails. extdebug If set. page 79. If set.56
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checkjobs If set. The ‘-F’ option to the declare builtin (see Section 4. the next command is skipped and not executed.1 with respect to quoted arguments to the conditional command’s =~ operator.

and subshells invoked with ( command ) inherit the ERROR trap. This option is enabled by default. the results of history substitution are not immediately passed to the shell parser. See Section 5. huponexit If set. If set.5. only directories and subdirectories match. histreedit If set. page 110). patterns which fail to match filenames during filename expansion result in an expansion error. If set. shell functions. a user is given the opportunity to re-edit a failed history substitution. the extended pattern matching features described above (see Section 3. for a description of FIGNORE. shell error messages are written in the standard gnu error message format. the pattern ‘**’ used in a filename expansion context will match a files and zero or more directories and subdirectories. If set.
failglob
force_fignore If set.6 [Signals]. page 32).2 [Bash Variables]. page 61. rather than overwriting the file. the history list is appended to the file named by the value of the HISTFILE variable when the shell exits. the resulting line is loaded into the Readline editing buffer. globstar If set. If the pattern is followed by a ‘/’. This option is enabled by default. the suffixes specified by the FIGNORE shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. page 24) are enabled.1 [Pattern Matching]. $’string’ and $"string" quoting is performed within ${parameter} expansions enclosed in double quotes. and Readline is being used. Instead. This option is enabled by default. Error tracing is enabled: command substitution. hostcomplete If set. Bash will attempt to perform hostname completion when a word containing a ‘@’ is being completed (see Section 8.8.
gnu_errfmt
. allowing further modification. and Readline is being used. Bash will send SIGHUP to all jobs when an interactive login shell exits (see Section 3.4. and Readline is being used. histappend If set. histverify If set.7.6 [Commands For Completion].Chapter 4: Shell Builtin Commands
57
6. extglob extquote If set.

page 71).
promptvars
. nocasematch If set. Bash matches filenames in a case-insensitive fashion when performing filename expansion. and Readline is being used. nocaseglob If set. Bash will not attempt to search the PATH for possible completions when completion is attempted on an empty line. Bash allows filename patterns which match no files to expand to a null string.1 [Invoking Bash]. mailwarn If set. The value may not be changed. lithist If enabled.58
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interactive_comments Allow a word beginning with ‘#’ to cause that word and all remaining characters on that line to be ignored in an interactive shell.10 [The Restricted Shell]. and quote removal after being expanded as described below (see Section 6. restricted_shell The shell sets this option if it is started in restricted mode (see Section 6.
login_shell The shell sets this option if it is started as a login shell (see Section 6. prompt strings undergo parameter expansion. rather than themselves. This is not reset when the startup files are executed. page 84). This option is enabled by default. If set. page 115) are enabled. allowing the startup files to discover whether or not a shell is restricted.6 [Programmable Completion]. page 82). This option is enabled by default. The value may not be changed. command substitution. and a file that Bash is checking for mail has been accessed since the last time it was checked. If set. and the cmdhist option is enabled. nullglob progcomp If set.9 [Printing a Prompt]. arithmetic expansion.
no_empty_cmd_completion If set. multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible. This option is enabled by default. Bash matches patterns in a case-insensitive fashion when performing matching while executing case or [[ conditional commands. the message "The mail in mailfile has been read" is displayed. the programmable completion facilities (see Section 8.

11 [Bash POSIX Mode]. nonzero otherwise. The Bash posix mode is described in Section 6. When setting or unsetting options. the return status is zero unless an optname is not a valid shell option. the special builtins differ from other builtin commands in three respects: 1.
The return status when listing options is zero if all optnames are enabled. the shift builtin prints an error message when the shift count exceeds the number of positional parameters. These are the posix special builtins: break : .
4. 2.Chapter 4: Shell Builtin Commands
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shift_verbose If this is set. xpg_echo If set. Assignment statements preceding the command stay in effect in the shell environment after the command completes. Special builtins are found before shell functions during command lookup. the echo builtin expands backslash-escape sequences by default. This option is enabled by default. a non-interactive shell exits. the source builtin uses the value of PATH to find the directory containing the file supplied as an argument. continue eval exec exit export readonly return set shift trap unset
. sourcepath If set. If a special builtin returns an error status. the posix standard has classified several builtin commands as special. 3. page 84. these builtins behave no differently than the rest of the Bash builtin commands. When Bash is executing in posix mode.4 Special Builtins
For historical reasons. When Bash is not executing in posix mode.

.

9 [Printing a Prompt]. A list of characters that separate fields. BASH The full pathname used to execute the current instance of Bash. or as an initial or trailing colon. page 92). A colon-separated list of directories in which the shell looks for commands. Each list entry can specify the message that is printed when new mail arrives in the mail file by separating the file name from the message with a ‘?’. If this parameter is set to a filename and the MAILPATH variable is not set.
IFS MAIL MAILPATH
OPTARG OPTIND PATH
PS1
PS2
5.5. The value of the last option argument processed by the getopts builtin. A colon-separated list of filenames which the shell periodically checks for new mail.
. $_ expands to the name of the current mail file. The index of the last option argument processed by the getopts builtin. See Section 6. Bash automatically assigns default values to a number of variables. When used in the text of the message.
5. page 19).3 [Job Control Variables]. A null directory name may appear as two adjacent colons.2 Bash Variables
These variables are set or used by Bash.1 Bourne Shell Variables
Bash uses certain shell variables in the same way as the Bourne shell.Chapter 5: Shell Variables
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5 Shell Variables
This chapter describes the shell variables that Bash uses. The default value is ‘> ’. The primary prompt string. The current user’s home directory. CDPATH HOME A colon-separated list of directories used as a search path for the cd builtin command. Bash assigns a default value to the variable. page 82. The default value is ‘\s-\v\$ ’. A zero-length (null) directory name in the value of PATH indicates the current directory. In some cases. Bash informs the user of the arrival of mail in the specified file. A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see Section 7. the default for the cd builtin command. but other shells do not normally treat them specially. for the complete list of escape sequences that are expanded before PS1 is displayed.2 [Tilde Expansion]. The value of this variable is also used by tilde expansion (see Section 3. The secondary prompt string. used when the shell splits words as part of expansion.

This variable is readonly. BASH_CMDS An associative array variable whose members correspond to the internal hash table of commands as maintained by the hash builtin (see Section 4.1 [Bourne Shell Builtins]. When a subroutine is executed.
. The options appearing in BASHOPTS are those reported as ‘on’ by ‘shopt’. This differs from $$ under certain circumstances.
BASH_EXECUTION_STRING The command argument to the ‘-c’ invocation option. If this variable is in the environment when Bash starts up. BASH_ENV If this variable is set when Bash is invoked to execute a shell script. BASH_ARGC An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The shell sets BASH_ARGC only when in extended debugging mode (see Section 4. page 55 for a description of the extdebug option to the shopt builtin).2 [The Shopt Builtin]. The shell sets BASH_ARGV only when in extended debugging mode (see Section 4.1 [Bourne Shell Builtins]. The final parameter of the last subroutine call is at the top of the stack. The number of parameters to the current subroutine (shell function or script executed with .3.62
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BASHOPTS
A colon-separated list of enabled shell options.3.2 [The Shopt Builtin]. page 55 for a description of the extdebug option to the shopt builtin). See Section 6. Each word in the list is a valid argument for the ‘-s’ option to the shopt builtin command (see Section 4. BASH_ARGV An array variable containing all of the parameters in the current bash execution call stack.3. the first parameter of the initial call is at the bottom. unsetting array elements cause commands to be removed from the hash table. BASH_COMMAND The command currently being executed or about to be executed. Elements added to this array appear in the hash table. each shell option in the list will be enabled before reading any startup files. the parameters supplied are pushed onto BASH_ARGV. page 35). its value is expanded and used as the name of a startup file to read before executing the script. Expands to the process id of the current Bash process. or source) is at the top of the stack. When a subroutine is executed. page 35). page 73.2 [Bash Startup Files]. unless the shell is executing a command as the result of a trap. the number of parameters passed is pushed onto BASH_ARGC.
BASHPID
BASH_ALIASES An associative array variable whose members correspond to the internal list of aliases as maintained by the alias builtin (see Section 4. such as subshells that do not require Bash to be re-initialized. page 55). in which case it is the command executing at the time of the trap. unsetting array elements cause aliases to be removed from the alias list. Elements added to this array appear in the alias list.2 [The Shopt Builtin].

page 80) whose members hold version information for this instance of Bash. Use LINENO to obtain the current line number.2 [Conditional Constructs]. This allows tracing output to be separated from diagnostic and error messages. ${BASH_LINENO[$i]} is the line number in the source file where ${FUNCNAME[$i]} was called (or ${BASH_ LINENO[$i-1]} if referenced within another shell function). This variable is read-only. BASH_XTRACEFD If set to an integer corresponding to a valid file descriptor.2. The values assigned to the array members are as follows: BASH_VERSINFO[0] The major version number (the release ). BASH_VERSINFO[4] The release status (e. BASH_VERSINFO[5] The value of MACHTYPE. The file descriptor is closed when BASH_XTRACEFD is unset or assigned a new value. Bash will write the trace output generated when ‘set -x’ is enabled to that file descriptor. page 10).. beta1 ). BASH_REMATCH An array variable whose members are assigned by the ‘=~’ binary operator to the [[ conditional command (see Section 3.4.
. The element with index n is the portion of the string matching the nth parenthesized subexpression. BASH_VERSINFO[3] The build version. BASH_VERSINFO A readonly array variable (see Section 6. BASH_VERSION The version number of the current instance of Bash.Chapter 5: Shell Variables
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BASH_LINENO An array variable whose members are the line numbers in source files corresponding to each member of FUNCNAME. BASH_SUBSHELL Incremented by one each time a subshell or subshell environment is spawned. BASH_VERSINFO[2] The patch level.7 [Arrays]. BASH_VERSINFO[1] The minor version number (the version).g. BASH_SOURCE An array variable whose members are the source filenames corresponding to the elements in the FUNCNAME array variable. The corresponding source file name is ${BASH_SOURCE[$i]}. The initial value is 0. The element with index 0 is the portion of the string matching the entire regular expression.

This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Section 8. Automatically set upon receipt of a SIGWINCH. even if it is subsequently reset. This variable is available only in shell functions invoked by the programmable completion facilities (see Section 8. COMP_KEY The key (or final key of a key sequence) used to invoke the current completion function. for listing alternatives on partial word completion.
. ‘@’. page 115). for normal completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Section 8. If the current cursor position is at the end of the current command. Used by the select builtin command to determine the terminal width when printing selection lists. page 115). page 115).6 [Programmable Completion].
COMP_WORDBREAKS The set of characters that the Readline library treats as word separators when performing word completion.6 [Programmable Completion]. for listing completions after successive tabs. Note that setting BASH_XTRACEFD to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed. COMP_LINE The current command line. COMP_POINT The index of the current cursor position relative to the beginning of the current command. COLUMNS COMP_CWORD An index into ${COMP_WORDS} of the word containing the current cursor position. page 115). COMP_WORDS An array variable consisting of the individual words in the current command line. The line is split into words as Readline would split it. it loses its special properties. page 115). the value of this variable is equal to ${#COMP_LINE}. This variable is available only in shell functions invoked by the programmable completion facilities (see Section 8.6 [Programmable Completion]. using COMP_ WORDBREAKS as described above. ‘!’. or ‘%’. for menu completion. to list completions if the word is not unmodified.6 [Programmable Completion]. ‘?’.64
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Unsetting BASH_XTRACEFD or assigning it the empty string causes the trace output to be sent to the standard error. COMP_TYPE Set to an integer value corresponding to the type of completion attempted that caused a completion function to be called: TAB. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Section 8. If COMP_WORDBREAKS is unset.6 [Programmable Completion].

If DIRSTACK is unset. The history comment character causes history substitution to be skipped for the remaining words on the line. The element with index 0 is the name of any currentlyexecuting shell function. Assigning to members of this array variable may be used to modify directories already in the stack. even if it is subsequently reset. it loses its special properties. page 123). The second character is the character which signifies ‘quick substitution’ when seen as the first character on a line. it is removed from the list of matches.6 [Programmable Completion]. This variable exists only when a shell function is executing. If FUNCNAME is unset. usually ‘#’. A colon-separated list of suffixes to ignore when performing filename completion. This variable is readonly. A sample value is ‘.
EMACS
EUID FCEDIT FIGNORE
FUNCNAME
GLOBIGNORE
histchars
. even if it is subsequently reset. normally ‘^’. If GROUPS is unset. Assignments to FUNCNAME have no effect and return an error status. The numeric effective user id of the current user. Assignment to this variable will not change the current directory. but the pushd and popd builtins must be used to add and remove directories. A colon-separated list of patterns defining the set of filenames to be ignored by filename expansion. that is. quick substitution. it loses its special properties. Directories appear in the stack in the order they are displayed by the dirs builtin. The optional third character is the character which indicates that the remainder of the line is a comment when found as the first character of a word. page 115). GROUPS An array variable containing the list of groups of which the current user is a member. The first character is the history expansion character. It does not necessarily cause the shell parser to treat the rest of the line as a comment. If Bash finds this variable in the environment when the shell starts with value ‘t’. A file name whose suffix matches one of the entries in FIGNORE is excluded from the list of matched file names. normally ‘!’. Assignments to GROUPS have no effect and return an error status. The editor used as a default by the ‘-e’ option to the fc builtin command. it assumes that the shell is running in an emacs shell buffer and disables line editing. even if it is subsequently reset.o:~’ An array variable containing the names of all shell functions currently in the execution call stack. Up to three characters which control history expansion. the character which signifies the start of a history expansion.Chapter 5: Shell Variables
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COMPREPLY An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see Section 8. DIRSTACK An array variable containing the current contents of the directory stack. If a filename matched by a filename expansion pattern also matches one of the patterns in GLOBIGNORE.3 [History Interaction]. it loses its special properties. and tokenization (see Section 9. The bottom-most element is "main".

The second and subsequent lines of a multi-line compound command are not tested. and a pattern of ‘[ ]*’ is identical to ignorespace. The default value is ‘~/. and are added to the history regardless of the value of HISTCONTROL. HISTIGNORE A colon-separated list of patterns used to decide which command lines should be saved on the history list. A value of ‘ignoredups’ causes lines which match the previous history entry to not be saved. HISTFILE The name of the file to which the command history is saved. A value of ‘ignoreboth’ is shorthand for ‘ignorespace’ and ‘ignoredups’. ‘&’ matches the previous history line.
HISTTIMEFORMAT If this variable is set and not null. Combining these two patterns. A value of ‘erasedups’ causes all previous lines matching the current line to be removed from the history list before that line is saved. all lines read by the shell parser are saved on the history list. or does not include a valid value. The default value is 500. the history file is truncated. even if it is subsequently reset. The second and subsequent lines of a multi-line compound command are not tested. the backslash is removed before attempting a match. of the current command. Each pattern is tested against the line after the checks specified by HISTCONTROL are applied. Any value not in the above list is ignored. its value is used as a format string for strftime to print the time stamp associated with each history entry displayed by the history builtin. subject to the value of HISTIGNORE. This uses the history comment character to distinguish timestamps from other history lines. HISTSIZE The maximum number of commands to remember on the history list. In addition to the normal shell pattern matching characters. If HISTCMD is unset. If this variable is set. lines which begin with a space character are not saved in the history list. provides the functionality of ignoreboth.
HISTCONTROL A colon-separated list of values controlling how commands are saved on the history list. The default value is 500.bash_history’. by removing the oldest entries. if necessary.
HISTFILESIZE The maximum number of lines contained in the history file. to contain no more than that number of lines. time stamps are written to the history file so they may be preserved across shell sessions. The history file is also truncated to this size after writing it when an interactive shell exits. If the list of values includes ‘ignorespace’. separating them with a colon.
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HISTCMD
The history number. and are added to the history regardless of the value of HISTIGNORE. When this variable is assigned a value. If HISTCONTROL is unset. HISTIGNORE subsumes the function of HISTCONTROL. ‘&’ may be escaped using a backslash. A pattern of ‘&’ is identical to ignoredups. or index in the history list. it loses its special properties. Each pattern is anchored at the beginning of the line and must match the complete line (no implicit ‘*’ is appended).

Used to determine the locale category for any category not specifically selected with a variable starting with LC_. If set. Controls the action of the shell on receipt of an EOF character as the sole input.inputrc’. the hostname list is cleared. overriding the default of ‘~/.5. A string that fully describes the system type on which Bash is executing. page 24). and collating sequences within filename expansion and pattern matching (see Section 3. page 24). If HOSTFILE is set. or does not name a readable file. If the variable does not exist. Automatically set upon receipt of a SIGWINCH. When HOSTFILE is unset. Bash attempts to read ‘/etc/hosts’ to obtain the list of possible hostname completions.2. then EOF signifies the end of input to the shell. This is only in effect for interactive shells.1.5 [Locale Translation]. the value denotes the number of consecutive EOF characters that can be read as the first character on an input line before the shell will exit.8 [Filename Expansion].Chapter 5: Shell Variables
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HOSTFILE
Contains the name of a file in the same format as ‘/etc/hosts’ that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running. but has no value.
HOSTNAME HOSTTYPE IGNOREEOF
INPUTRC LANG LC_ALL LC_COLLATE
The name of the Readline initialization file. equivalence classes. The name of the current host. and determines the behavior of range expressions.5.
. Bash adds the contents of the new file to the existing list.
LC_CTYPE
This variable determines the interpretation of characters and the behavior of character classes within filename expansion and pattern matching (see Section 3. This variable overrides the value of LANG and any other LC_ variable specifying a locale category.
LC_MESSAGES This variable determines the locale used to translate double-quoted strings preceded by a ‘$’ (see Section 3. Used by the select builtin command to determine the column length for printing selection lists. If the variable exists but does not have a numeric value (or has no value) then the default is 10. A string describing the machine Bash is running on. This variable determines the collation order used when sorting the results of filename expansion. the next time hostname completion is attempted after the value is changed. LINENO LINES MACHTYPE The line number in the script or shell function currently executing. in the standard gnu cpu-company-system format. LC_NUMERIC This variable determines the locale category used for number formatting. page 7).8 [Filename Expansion].

bash enables posix mode. If set to the value 1. page 80) containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command). The previous working directory as set by the cd builtin. This variable is readonly. The default is ‘+ ’.3. The current working directory as set by the cd builtin. the shell enters posix mode (see Section 6. to indicate multiple levels of indirection. PPID The process id of the shell’s parent process. OLDPWD OPTERR OSTYPE PIPESTATUS An array variable (see Section 6. Characters removed are replaced with an ellipsis.7 [Arrays]. the shell disables mail checking. or set to a value that is not a number greater than or equal to zero.11 [Bash POSIX Mode]. The default variable for the read builtin. PROMPT_DIRTRIM If set to a number greater than zero. Assigning a value to this variable seeds the random number generator. POSIXLY_CORRECT If this variable is in the environment when bash starts. When it is time to check for mail.9 [Printing a Prompt].1 [The Set Builtin]. Each time this parameter is referenced. the value is interpreted as a command to execute before the printing of each primary prompt ($PS1). page 82).
PROMPT_COMMAND If set.68
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MAILCHECK How often (in seconds) that the shell should check for mail in the files specified in the MAILPATH or MAIL variables. The default is 60 seconds. as necessary. If this variable is unset. The first character of PS4 is replicated multiple times. as if the ‘--posix’ invocation option had been supplied. as if the command set -o posix had been executed. the shell does so before displaying the primary prompt. Bash displays error messages generated by the getopts builtin command. page 51). If this variable is not set. the value is used as the number of trailing directory components to retain when expanding the \w and \W prompt string escapes (see Section 6.
PWD RANDOM
REPLY
. a random integer between 0 and 32767 is generated. If it is set while the shell is running. page 84) before reading the startup files. A string describing the operating system Bash is running on. the select command prompts with ‘#? ’ The value is the prompt printed before the command line is echoed when the ‘-x’ option is set (see Section 4. PS3 PS4 The value of this variable is used as the prompt for the select command.

The ‘%’ character introduces an escape sequence that is expanded to a time value or other information.
SHELL
SHELLOPTS
SHLVL TIMEFORMAT
Incremented by one each time a new instance of Bash is started.1 [The Set Builtin]. Bash assigns to it the full pathname of the current user’s login shell. TMOUT If set to a value greater than zero. Bash acts as if it had the value $’\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS’ If the value is null. A colon-separated list of enabled shell options. Each word in the list is a valid argument for the ‘-o’ option to the set builtin command (see Section 4.2 [Bash Builtins]. The optional l specifies a longer format.
The optional p is a digit specifying the precision.
. the value 3 is used. the number of fractional digits after a decimal point. computed as (%U + %S) / %R. including minutes. A trailing newline is added when the format string is displayed. A value of 0 causes no decimal point or fraction to be output. The escape sequences and their meanings are as follows. The CPU percentage. no timing information is displayed. The select command (see Section 3.3. Assignment to this variable resets the count to the value assigned. page 10) terminates if input does not arrive after TMOUT seconds when input is coming from a terminal. values of p greater than 3 are changed to 3. The number of CPU seconds spent in user mode.4. The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed.FF s. %% %[p][l]R %[p][l]U %[p][l]S %P A literal ‘%’. The full pathname to the shell is kept in this environment variable. The number of CPU seconds spent in system mode. the braces denote optional portions. TMOUT is treated as the default timeout for the read builtin (see Section 4. If this variable is not set. and the expanded value becomes the value assigned plus the number of seconds since the assignment.2. This is intended to be a count of how deeply your Bash shells are nested. At most three places after the decimal point may be specified. The elapsed time in seconds. If this variable is in the environment when Bash starts up. each shell option in the list will be enabled before reading any startup files. The options appearing in SHELLOPTS are those reported as ‘on’ by ‘set -o’. This variable is readonly.2 [Conditional Constructs]. If it is not set when the shell starts.Chapter 5: Shell Variables
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SECONDS
This variable expands to the number of seconds since the shell was started. page 41). If p is not specified. of the form MM mSS. page 51). The value of p determines whether or not the fraction is included.

Bash uses its value as the name of a directory in which Bash creates temporary files for the shell’s use.
. This variable is readonly.70
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In an interactive shell. The numeric real user id of the current user. Bash terminates after that number of seconds if input does not arrive. the value is interpreted as the number of seconds to wait for input after issuing the primary prompt when the shell is interactive. TMPDIR UID If set.

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6 Bash Features
This section describes features unique to Bash.

6.1 Invoking Bash
bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...] bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] -c string [argument ...] bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...] In addition to the single-character shell command-line options (see Section 4.3.1 [The Set Builtin], page 51), there are several multi-character options that you can use. These options must appear on the command line before the single-character options to be recognized. --debugger Arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see Section 4.3.2 [The Shopt Builtin], page 55 for a description of the extdebug option to the shopt builtin) and shell function tracing (see Section 4.3.1 [The Set Builtin], page 51 for a description of the -o functrace option). --dump-po-strings A list of all double-quoted strings preceded by ‘$’ is printed on the standard output in the gnu gettext PO (portable object) file format. Equivalent to ‘-D’ except for the output format. --dump-strings Equivalent to ‘-D’. --help Display a usage message on standard output and exit successfully.

--noediting Do not use the gnu Readline library (see Chapter 8 [Command Line Editing], page 93) to read command lines when the shell is interactive. --noprofile Don’t load the system-wide startup file ‘/etc/profile’ or any of the personal initialization files ‘~/.bash_profile’, ‘~/.bash_login’, or ‘~/.profile’ when Bash is invoked as a login shell. --norc Don’t read the ‘~/.bashrc’ initialization file in an interactive shell. This is on by default if the shell is invoked as sh.

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--posix

Change the behavior of Bash where the default operation differs from the posix standard to match the standard. This is intended to make Bash behave as a strict superset of that standard. See Section 6.11 [Bash POSIX Mode], page 84, for a description of the Bash posix mode.

--restricted Make the shell a restricted shell (see Section 6.10 [The Restricted Shell], page 84). --verbose Equivalent to ‘-v’. Print shell input lines as they’re read. --version Show version information for this instance of Bash on the standard output and exit successfully. There are several single-character options that may be supplied at invocation which are not available with the set builtin. -c string Read and execute commands from string after processing the options, then exit. Any remaining arguments are assigned to the positional parameters, starting with $0. -i -l Force the shell to run interactively. Interactive shells are described in Section 6.3 [Interactive Shells], page 75. Make this shell act as if it had been directly invoked by login. When the shell is interactive, this is equivalent to starting a login shell with ‘exec -l bash’. When the shell is not interactive, the login shell startup files will be executed. ‘exec bash -l’ or ‘exec bash --login’ will replace the current shell with a Bash login shell. See Section 6.2 [Bash Startup Files], page 73, for a description of the special behavior of a login shell. Make the shell a restricted shell (see Section 6.10 [The Restricted Shell], page 84). If this option is present, or if no arguments remain after option processing, then commands are read from the standard input. This option allows the positional parameters to be set when invoking an interactive shell. A list of all double-quoted strings preceded by ‘$’ is printed on the standard output. These are the strings that are subject to language translation when the current locale is not C or POSIX (see Section 3.1.2.5 [Locale Translation], page 7). This implies the ‘-n’ option; no commands will be executed.

-r -s

-D

[-+]O [shopt_option] shopt option is one of the shell options accepted by the shopt builtin (see Section 4.3.2 [The Shopt Builtin], page 55). If shopt option is present, ‘-O’ sets the value of that option; ‘+O’ unsets it. If shopt option is not supplied, the names and values of the shell options accepted by shopt are printed on the standard output. If the invocation option is ‘+O’, the output is displayed in a format that may be reused as input.

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--

A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments.

A login shell is one whose first character of argument zero is ‘-’, or one invoked with the ‘--login’ option. An interactive shell is one started without non-option arguments, unless ‘-s’ is specified, without specifying the ‘-c’ option, and whose input and output are both connected to terminals (as determined by isatty(3)), or one started with the ‘-i’ option. See Section 6.3 [Interactive Shells], page 75, for more information. If arguments remain after option processing, and neither the ‘-c’ nor the ‘-s’ option has been supplied, the first argument is assumed to be the name of a file containing shell commands (see Section 3.8 [Shell Scripts], page 33). When Bash is invoked in this fashion, $0 is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash’s exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0.

6.2 Bash Startup Files
This section describes how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in file names as described above under Tilde Expansion (see Section 3.5.2 [Tilde Expansion], page 19). Interactive shells are described in Section 6.3 [Interactive Shells], page 75.

Invoked as an interactive login shell, or with ‘--login’
When Bash is invoked as an interactive login shell, or as a non-interactive shell with the ‘--login’ option, it first reads and executes commands from the file ‘/etc/profile’, if that file exists. After reading that file, it looks for ‘~/.bash_profile’, ‘~/.bash_login’, and ‘~/.profile’, in that order, and reads and executes commands from the first one that exists and is readable. The ‘--noprofile’ option may be used when the shell is started to inhibit this behavior. When a login shell exits, Bash reads and executes commands from the file ‘~/.bash_logout’, if it exists.

Invoked as an interactive non-login shell
When an interactive shell that is not a login shell is started, Bash reads and executes commands from ‘~/.bashrc’, if that file exists. This may be inhibited by using the ‘--norc’ option. The ‘--rcfile file’ option will force Bash to read and execute commands from file instead of ‘~/.bashrc’. So, typically, your ‘~/.bash_profile’ contains the line if [ -f ~/.bashrc ]; then . ~/.bashrc; fi after (or before) any login-specific initializations.

Invoked non-interactively
When Bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the

while conforming to the posix standard as well.
Invoked with name sh
If Bash is invoked with the name sh.bashrc’. or the secure shell daemon sshd. but the effective user id is not reset. no startup files are read. CDPATH. it reads and executes commands from ‘~/. if they appear in the environment. and GLOBIGNORE variables. When invoked as sh. and uses the expanded value as the name of a file to read and execute. Bash enters posix mode after the startup files are read. are ignored. Bash looks for the variable ENV. usually rshd. "$BASH_ENV". but rshd does not generally invoke the shell with those options or allow them to be specified. the ‘--rcfile’ option has no effect. it first attempts to read and execute commands from ‘/etc/profile’ and ‘~/. Bash behaves as if the following command were executed: if [ -n "$BASH_ENV" ].profile’. and the ‘--rcfile’ option may be used to force another file to be read.
Invoked in posix mode
When Bash is started in posix mode. No other startup files are read. and the -p option is not supplied.
Invoked by remote shell daemon
Bash attempts to determine when it is being run with its standard input connected to a a network connection. In this mode. It will not do this if invoked as sh.
Invoked with unequal effective and real uid/gids
If Bash is started with the effective user (group) id not equal to the real user (group) id. and the effective user id is set to the real user id. When invoked as an interactive login shell. if a non-interactive shell is invoked with the ‘--login’ option. If Bash determines it is being run in this fashion. as if by the remote shell daemon. it follows the posix standard for startup files.
. interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. expands its value if it is defined. or as a non-interactive shell with the ‘--login’ option. fi but the value of the PATH variable is not used to search for the file name. the startup behavior is the same. Bash attempts to read and execute commands from the login shell startup files. it tries to mimic the startup behavior of historical versions of sh as closely as possible. in that order.74
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expanded value as the name of a file to read and execute. The ‘--noprofile’ option may be used to inhibit this behavior. shell functions are not inherited from the environment. A non-interactive shell invoked with the name sh does not attempt to read any other startup files. If the -p option is supplied at invocation. if that file exists and is readable. The ‘--norc’ option may be used to inhibit this behavior. When invoked as an interactive shell with the name sh. then . as with the ‘--posix’ command line option. BASHOPTS. As noted above. Since a shell invoked as sh does not attempt to read and execute commands from any other startup files. the SHELLOPTS.

page 121) and history expansion (see Section 9. 6.
6. unless ‘-s’ is specified.1 [Bash History Facilities].. For example: case "$-" in *i*) echo This shell is interactive . *) echo This shell is not interactive . page 51). 1. page 89) is enabled by default.3 Interactive Shells
6. When job control is in effect.3 [History Interaction]. page 123) are enabled by default. Bash executes the value of the PROMPT_COMMAND variable as a command before printing the primary prompt. test the value of the ‘-’ special parameter. Bash ignores the keyboard-generated job control signals SIGTTIN. and whose input and error output are both connected to terminals (as determined by isatty(3)). 4. and SIGTSTP. and expands and displays PS2 before reading the second and subsequent lines of a multi-line command. it is unset in non-interactive shells.. then echo This shell is not interactive else echo This shell is interactive fi
6. and set in interactive shells. page 73. esac Alternatively. $PS1 (see Section 5. SIGTTOU. An interactive shell generally reads from and writes to a user’s terminal. Readline (see Chapter 8 [Command Line Editing]. It contains i when the shell is interactive.1 What is an Interactive Shell?
An interactive shell is one started without non-option arguments. Thus: if [ -z "$PS1" ]. page 93) is used to read commands from the user’s terminal. or one started with the ‘-i’ option.1 [The Set Builtin]. Bash
. Command history (see Section 9. page 61). 3. without specifying the ‘-c’ option. Startup files are read and executed as described in Section 6. Bash inspects the value of the ignoreeof option to set -o instead of exiting immediately when it receives an EOF on its standard input when reading a command (see Section 4. The ‘-s’ invocation option may be used to set the positional parameters when an interactive shell is started.3.Chapter 6: Bash Features
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6.2 [Bash Startup Files]. 5. Bash expands and displays PS1 before reading the first line of a command. it changes its behavior in several ways.3.3. Job Control (see Chapter 7 [Job Control]. startup scripts may examine the variable PS1. 7.2 [Bash Variables]. 2.3.3 Interactive Shell Behavior
When the shell is running interactively.2 Is this Shell Interactive?
To determine within a startup script whether or not Bash is running interactively.

Simple spelling correction for directory arguments to the cd builtin is enabled by default (see the description of the cdspell option to the shopt builtin in Section 4. 19. Alias expansion (see Section 6.7. If the file argument to one of the primaries is one of ‘/dev/stdin’. 14.
21.6 [Aliases].6 [Signals]. 16. 10. 17.4 Bash Conditional Expressions
Conditional expressions are used by the [[ compound command and the test and [ builtin commands. and MAILCHECK shell variables (see Section 5. page 35). ‘/dev/stdout’.7. An interactive login shell sends a SIGHUP to all jobs on exit if the huponexit shell option has been enabled (see Section 3. Bash ignores SIGTERM (see Section 3.2 [Bash Variables]. page 32). page 79) is performed by default. file descriptor 0. The shell will not exit on expansion errors caused by var being unset or null in ${var:?word} expansions (see Section 3. 20. 15. When running in posix mode. page 84). Redirection errors encountered by shell builtins will not cause the shell to exit.
will save the command history to the file named by $HISTFILE when an interactive shell exits.6 [Signals]. page 55).1 [Bourne Shell Builtins]. page 51). page 61). Parser syntax errors will not cause the shell to exit. 13.76
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8. SIGINT is caught and handled ((see Section 3.6 [Signals]. Expressions may be unary or binary. If the file argument to one of the primaries is of the form ‘/dev/fd/N’. 1. Unary expressions are often used to examine the status of a file. page 32). In the absence of any traps. 12.
6. Expansion errors due to references to unbound shell variables after ‘set -u’ has been enabled will not cause the shell to exit (see Section 4.7. The ‘-n’ invocation option is ignored.3. then file descriptor N is checked.3.1 [The Set Builtin]. MAILPATH.
. depending on the values of the MAIL. A failed exec will not cause the shell to exit (see Section 4. In the absence of any traps.3 [Shell Parameter Expansion]. page 51). Unless otherwise specified.1 [The Set Builtin]. respectively. and ‘set -n’ has no effect (see Section 4.11 [Bash POSIX Mode]. page 32). rather than the link itself. primaries that operate on files follow symbolic links and operate on the target of the link. When used with ‘[[’.2 [The Shopt Builtin]. There are string operators and numeric comparison operators as well. The ‘<’ and ‘>’ operators sort lexicographically using the current locale.5. 18. 11. or ‘/dev/stderr’.3.2 [Bash Variables]. page 19). SIGINT will interrupt some shell builtins. a special builtin returning an error status will not cause the shell to exit (see Section 6. 9. or 2. The shell will check the value of the TMOUT variable and exit if a command is not read within the specified number of seconds after printing $PS1 (see Section 5. Bash will check for mail periodically. -a file True if file exists. is checked. page 61).

True if file exists and is a symbolic link. True if file exists and its set-user-id bit is set. True if file exists and is writable. True if file exists and has been modified since it was last read. True if file exists and is a regular file. True if file exists and is a directory. True if file exists and is executable.
file1 -nt file2 True if file1 is newer (according to modification date) than file2. True if file descriptor fd is open and refers to a terminal. True if file exists and is a named pipe (FIFO).3. -z string True if the length of string is zero. file1 -ef file2 True if file1 and file2 refer to the same device and inode numbers. True if file exists and has a size greater than zero.Chapter 6: Bash Features
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-b file -c file -d file -e file -f file -g file -h file -k file -p file -r file -s file -t fd -u file -w file -x file -O file -G file -L file -S file -N file
True if file exists and is a block special file.
. True if file exists and is a character special file.1 [The Set Builtin]. page 51). True if file exists and is owned by the effective user id. The list of options appears in the description of the ‘-o’ option to the set builtin (see Section 4. True if file exists and is a symbolic link. True if file exists. True if file exists and is owned by the effective group id. True if file exists and is a socket. True if file exists and its "sticky" bit is set. True if file exists and is readable. or if file2 exists and file1 does not. file1 -ot file2 True if file1 is older than file2. -n string string True if the length of string is non-zero. True if file exists and its set-group-id bit is set. or if file1 exists and file2 does not. -o optname True if shell option optname is enabled.

string1 < string2 True if string1 sorts before string2 lexicographically. and values are the same as in the C language. as one of the shell expansions or by the let and the ‘-i’ option to the declare builtins. or greater than or equal to arg2. though division by 0 is trapped and flagged as an error. associativity. Evaluation is done in fixed-width integers with no check for overflow. ‘-lt’. ‘=’ should be used with the test command for posix conformance. respectively. not equal to. Arg1 and arg2 may be positive or negative integers. subtraction left and right bitwise shifts comparison equality and inequality bitwise AND bitwise exclusive OR bitwise OR logical AND logical OR
. string1 > string2 True if string1 sorts after string2 lexicographically. ‘-le’. string1 != string2 True if the strings are not equal. less than. less than or equal to.5 Shell Arithmetic
The shell allows arithmetic expressions to be evaluated. or ‘-ge’. The levels are listed in order of decreasing precedence. remainder addition. id++ id-. These arithmetic binary operators return true if arg1 is equal to. ‘-gt’. arg1 OP arg2 OP is one of ‘-eq’. division.
6.variable post-increment and post-decrement ++id --id variable pre-increment and pre-decrement -+ !~ ** */% +<< >> <= >= < > == != & ^ | && || unary minus and plus logical and bitwise negation exponentiation multiplication. ‘-ne’. The operators and their precedence. The following list of operators is grouped into levels of equal-precedence operators. greater than.78
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string1 == string2 string1 = string2 True if the strings are equal.

unless the expand_aliases shell option is set using shopt (see Section 4. the uppercase letters. parameter expansion is performed before the expression is evaluated. expr2 comma Shell variables are allowed as operands. ‘@’. This means that one may alias ls to "ls -F". If so. where base is a decimal number between 2 and 64 representing the arithmetic base. numbers take the form [base #]n. Aliases are created and listed with the alias command. then the next command word following the alias is also checked for alias expansion. A shell variable need not have its integer attribute turned on to be used in an expression.6 Aliases
Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. page 55). lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35. then base 10 is used. The value of a variable is evaluated as an arithmetic expression when it is referenced. and Bash does not try to recursively expand the replacement text. A null value evaluates to 0. Within an expression. in that order. or when a variable which has been given the integer attribute using ‘declare -i’ is assigned a value. and removed with the unalias command. Sub-expressions in parentheses are evaluated first and may override the precedence rules above. The shell maintains a list of aliases that may be set and unset with the alias and unalias builtin commands. if unquoted. and n is a number in that base. There is no mechanism for using arguments in the replacement text. ‘$’. shell variables may also be referenced by name without using the parameter expansion syntax. Operators are evaluated in order of precedence. a shell function should be used (see Section 3. ‘=’ and any of the shell metacharacters or quoting characters listed above may not appear in an alias name.
. is checked to see if it has an alias.Chapter 6: Bash Features
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expr ? expr : expr conditional operator = *= /= %= += -= <<= >>= &= ^= |= assignment expr1 . Constants with a leading 0 are interpreted as octal numbers. The first word of each simple command. If the last character of the alias value is a space or tab character. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The first word of the replacement text is tested for aliases.3 [Shell Functions]. Aliases are not expanded when the shell is not interactive. page 14).3. and ‘_’.2 [The Shopt Builtin]. The characters ‘/’. that word is replaced by the text of the alias. for instance. If base is less than or equal to 36. as in csh. The replacement text may contain any valid shell input. The digits greater than 9 are represented by the lowercase letters. If base # is omitted. Otherwise. but a word that is identical to an alias being expanded is not expanded a second time. including shell metacharacters. If arguments are needed. ‘‘’.
6. A leading ‘0x’ or ‘0X’ denotes hexadecimal.

nor any requirement that members be indexed or assigned contiguously. As a consequence. Arrays are assigned to using compound assignments of the form name=(value1 . if the optional subscript is supplied. and do not use alias in compound commands. that index is assigned to.5 [Shell Arithmetic]. An indexed array is created automatically if any variable is assigned to using the syntax name[subscript]=value The subscript is treated as an arithmetic expression that must evaluate to a number greater than or equal to zero. If the subscript is
. valuen) where each value is of the form [subscript]=string. Aliases are expanded when a command is read.
6. the subscript is required. For almost every purpose. Any element of an array may be referenced using ${name[subscript]}. This syntax is also accepted by the declare builtin. the declare builtin will explicitly declare an array.7 Arrays
Bash provides one-dimensional indexed and associative array variables. the subscript is ignored. Attributes may be specified for an array variable using the declare and readonly builtins. associative arrays use arbitrary strings. Associative arrays are created using declare -A name. Each attribute applies to all members of an array. Aliases are expanded when a function definition is read. To be safe. use declare -a name The syntax declare -a name[subscript] is also accepted. otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexed arrays are referenced using integers (including arithmetic expressions (see Section 6. When assigning to indexed arrays. not when it is executed. To explicitly declare an array. Indexed array assignments do not require the bracket and subscript. There is no maximum limit on the size of an array. This behavior is also an issue when functions are executed. Any variable may be used as an indexed array..80
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The rules concerning the definition and use of aliases are somewhat confusing. not when the function is executed. an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The braces are required to avoid conflicts with the shell’s filename expansion operators. Bash always reads at least one complete line of input before executing any of the commands on that line.. shell functions are preferred over aliases. always put alias definitions on a separate line. Individual array elements may be assigned to using the name[subscript]=value syntax introduced above. because a function definition is itself a compound command. page 78) and are zero-based. The commands following the alias definition on that line are not affected by the new alias. aliases defined in a function are not available until after that function is executed. When assigning to an associative array. Therefore. Indexing starts at zero.

6. An array variable is considered set if a subscript has been assigned a value. and readonly builtins each accept a ‘-a’ option to specify an indexed array and a ‘-A’ option to specify an associative array. the word expands to all members of the array name.
. Clears the directory stack by deleting all of the elements. These subscripts differ only when the word appears within double quotes. The declare. ${name[@]} expands to nothing.
6. and the expansion of the last parameter is joined with the last part of the original word. the expansion is the number of elements in the array. starting with zero. the expansion of the first parameter is joined with the beginning part of the original word. +N -N -c Displays the N th directory (counting from the left of the list printed by dirs when invoked without options). Referencing an array variable without a subscript is equivalent to referencing with a subscript of 0. The set and declare builtins display array values in a way that allows them to be reused as input. Directories are added to the list with the pushd command. unset name.8. and can read values from the standard input into individual array elements. the popd command removes directories from the list. The contents of the directory stack are also visible as the value of the DIRSTACK shell variable. starting with zero. The dirs builtin displays the contents of the directory stack. local. ${name[*]} expands to a single word with the value of each array member separated by the first character of the IFS variable. This is analogous to the expansion of the special parameters ‘@’ and ‘*’. If subscript is ‘@’ or ‘*’. and ${name[@]} expands each element of name to a separate word. removes the entire array. The unset builtin is used to destroy arrays. unset name [subscript] destroys the array element at index subscript. If the double-quoted expansion occurs within a word. The read builtin accepts a ‘-a’ option to assign a list of words read from the standard input to an array. If the word is double-quoted.Chapter 6: Bash Features
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‘@’ or ‘*’.1 Directory Stack Builtins
dirs dirs [+N | -N] [-clpv] Display the list of currently remembered directories. and the popd builtin removes specified directories from the stack and changes the current directory to the directory removed. Displays the N th directory (counting from the right of the list printed by dirs when invoked without options).8 The Directory Stack
The directory stack is a list of recently-visited directories. where name is an array. ${#name[subscript]} expands to the length of ${name[subscript]}. Care must be taken to avoid unwanted side effects caused by filename expansion. When there are no array members. A subscript of ‘*’ or ‘@’ also removes the entire array. The pushd builtin adds directories to the stack as it changes the current directory. The null string is a valid value.

then the value is executed just as if it had been typed on the command line. so that only the stack is manipulated. When no arguments are given. Makes the current working directory be the top of the stack.
popd [+N | -N] [-n] Remove the top entry from the directory stack. With no arguments.
-N
dir
6. Causes dirs to print the directory stack with one entry per line. Causes dirs to print the directory stack with one entry per line. Brings the N th directory (counting from the left of the list printed by dirs. starting with zero) to the top of the list by rotating the stack. starting with zero. +N -N -n pushd pushd [-n] [+N | -N | dir ] Save the current directory on the top of the directory stack and then cd to dir. popd removes the top directory from the stack and performs a cd to the new top directory..9 Controlling the Prompt
The value of the variable PROMPT_COMMAND is examined just before Bash prints each primary prompt.82
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-l -p -v popd
Produces a longer listing. so that only the stack is manipulated. Suppresses the normal change of directory when removing directories from the stack. Brings the N th directory (counting from the right of the list printed by dirs. Removes the N th directory (counting from the right of the list printed by dirs). Removes the N th directory (counting from the left of the list printed by dirs). starting with zero. the following table describes the special characters which can appear in the prompt variables: \a A bell character. i. pushd exchanges the top two directories. In addition. and then executes the equivalent of ‘cd dir ’. and cd to the new top directory. If PROMPT_COMMAND is set and has a non-null value. popd is equivalent to popd +0. The elements are numbered from 0 starting at the first directory listed with dirs. the default listing format uses a tilde to denote the home directory. cds to dir.e. -n +N Suppresses the normal change of directory when adding directories to the stack. starting with zero) to the top of the list by rotating the stack. prefixing each entry with its index in the stack.
.

This could be used to embed a terminal control sequence into the prompt. The command number of this command. The time. Begin a sequence of non-printing characters. The time.g. page 121). A backslash.0) The current working directory. in "Weekday Month Date" format (e. A newline.Chapter 6: Bash Features
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\d \D{format}
The date. The username of the current user. A carriage return. The format is passed to strftime(3) and the result is inserted into the prompt string..g.. The hostname. If the effective uid is 0. with $HOME abbreviated with a tilde. The name of the shell. The version of Bash (e.00. in 12-hour am/pm format. The basename of the shell’s terminal device name. an empty format results in a locale-specific time representation. in 24-hour HH:MM:SS format. The hostname.00) The release of Bash. The history number of this command. which may include commands restored from the history file (see Section 9.
\e \h \H \j \l \n \r \s \t \T \@ \A \u \v \V \w \W \! \# \$ \nnn \\ \[ \]
An escape character.g. 2. version + patchlevel (e. otherwise $.
The command number and the history number are usually different: the history number of a command is its position in the history list. End a sequence of non-printing characters. The basename of $PWD. in 12-hour HH:MM:SS format. up to the first ‘. in 24-hour HH:MM format. while the command number is the position in the sequence of commands executed during the current shell session.
.. 2. the basename of $0 (the portion following the final slash). The time. The braces are required.1 [Bash History Facilities].’. "Tue May 26"). with $HOME abbreviated with a tilde (uses the $PROMPT_DIRTRIM variable). The number of jobs currently managed by the shell. The character whose ASCII code is the octal value nnn. #. The time.

where signame is. page 33). PATH. builtin command. The message printed by the job control code and builtins when a job is stopped is ‘Stopped(signame )’. • Using the enable builtin command to enable disabled shell builtins. and ‘>>’ redirection operators. • Specifying a filename containing a slash as an argument to the ‘-p’ option to the hash builtin command. When a command that is found to be a shell script is executed (see Section 3. • Redirecting output using the ‘>’. • Specifying command names containing slashes. and quote removal. The following list is what’s changed when ‘posix mode’ is in effect: 1. When a command in the hash table no longer exists. • Importing function definitions from the shell environment at startup. arithmetic expansion. • Adding or deleting builtin commands with the ‘-f’ and ‘-d’ options to the enable builtin. or the ‘--restricted’ or ‘-r’ option is supplied at invocation.11 Bash POSIX Mode
Starting Bash with the ‘--posix’ command-line option or executing ‘set -o posix’ while Bash is running will cause Bash to conform more closely to the posix standard by changing the behavior to match that specified by posix in areas where the Bash default differs. 3.10 The Restricted Shell
If Bash is started with the name rbash. Bash enters posix mode after reading the startup files. the shell becomes restricted. • Parsing the value of SHELLOPTS from the shell environment at startup. rbash turns off any restrictions in the shell spawned to execute the script. A restricted shell is used to set up an environment more controlled than the standard shell. A restricted shell behaves identically to bash with the exception that the following are disallowed or not performed: • Changing directories with the cd builtin. ENV. it is expanded via parameter expansion. 2. Bash will re-search $PATH to find the new location. • Specifying the ‘-p’ option to the command builtin. ‘<>’. • Setting or unsetting the values of the SHELL. page 41).84
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After the string is decoded.2 [Bash Builtins]. • Using the exec builtin to replace the shell with another command. subject to the value of the promptvars shell option (see Section 4. These restrictions are enforced after any startup files are read. When invoked as sh. ‘>|’.8 [Shell Scripts]. command substitution. ‘&>’.
6. SIGTSTP. The message printed by the job control code and builtins when a job exits with a non-zero status is ‘Done(status)’. ‘>&’. for example. or BASH_ENV variables. This is also available with ‘shopt -s checkhash’. • Turning off restricted mode with ‘set +r’ or ‘set +o restricted’.
6.
. • Specifying a filename containing a slash as an argument to the .

which does not include an indication of whether the job is the current or previous job.Chapter 6: Bash Features
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4. The fatal errors are those listed in the POSIX standard. 15. If a posix special builtin returns an error status. 22. filename is not found. a non-interactive shell exits. posix special builtins are found before shell functions during command lookup. This means that cd will fail if no valid directory name can be constructed from any of the entries in $CDPATH. 23. Declaring a function with an invalid name causes a fatal syntax error in non-interactive shells.
. without the ‘SIG’ prefix. 12. The default history file is ‘~/. rather than on all assignment statements on the line. The kill builtin does not accept signal names with a ‘SIG’ prefix. Redirection operators do not perform word splitting on the word in the redirection. 11. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression. Non-interactive shells exit if filename in . as if a posix special builtin command had been executed. Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion. even if the a directory with the same name as the name given as an argument to cd exists in the current directory. If CDPATH is set. 9. and so on. 6. 19. 5. Process substitution is not available. and parameter expansion is performed on the values of PS1 and PS2 regardless of the setting of the promptvars option. Tilde expansion is only performed on assignments preceding a command name. and underscores. Assignment statements preceding posix special builtins persist in the shell environment after the builtin completes. 14. redirection errors. Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive. 24. The posix PS1 and PS2 expansions of ‘!’ to the history number and ‘!!’ to ‘!’ are enabled. 10. separated by spaces. 18. The bg builtin uses the required format to describe each job placed in the background. A variable assignment error occurs. 7. That is. digits.sh_history’ (this is the default value of $HISTFILE). 13. 8. they may not contain characters other than letters. variable assignment errors for assignments preceding the command name. A non-interactive shell exits with an error status if the iteration variable in a for statement or the selection variable in a select statement is a readonly variable. and include things like passing incorrect options. Function names must be valid shell names. 17. A non-interactive shell exits with an error status if a variable assignment error occurs when no command name follows the assignment statements. 16. for example. The output of ‘kill -l’ prints all the signal names on a single line. when trying to assign a value to a readonly variable. the cd builtin will not implicitly append the current directory to it. and may not start with a digit. Assignment statements preceding shell function calls persist in the shell environment after the function returns. The posix startup files are executed ($ENV) rather than the normal Bash files. 20. 21.

The vi editing mode will invoke the vi editor directly when the ‘v’ command is run. they should use ‘-’ as the first argument. even if it is not asked to check the file system with the ‘-P’ option. When the alias builtin displays alias definitions. 29. When the pwd builtin is supplied the ‘-P’ option. Subshells spawned to execute command substitutions inherit the value of the ‘-e’ option from the parent shell. The pwd builtin verifies that the value it prints is the same as the current directory. though the shell will attempt to execute such a file if it is the only so-named file found in $PATH. even if the result contains nonprinting characters. 34. 40. The export and readonly builtin commands display their output in the format required by posix. The .86
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25. 28. the fc builtin does not include an indication of whether or not a history entry has been modified. The arrival of SIGCHLD when a trap is set on SIGCHLD does not interrupt the wait builtin and cause it to return immediately. When not in posix mode. When the set builtin is invoked without options. The ulimit builtin uses a block size of 512 bytes for the ‘-c’ and ‘-f’ options. When listing the history. 35. 33. 41. unless they contain shell metacharacters. 36. even in non-interactive shells. The trap command is run once for each child that exits. If users want to reset the handler for a given signal to the original disposition. cd will fail instead of falling back to physical mode. Bash clears the ‘-e’ option in such subshells. 30. The trap builtin displays signal names without the leading SIG. it displays variable values without quotes. 43. and the pathname constructed from $PWD and the directory name supplied as an argument does not refer to an existing directory. When the set builtin is invoked without options. 38. after escape characters are converted. Each argument is displayed. 42. it resets $PWD to a pathname containing no symlinks. unless that argument consists solely of digits and is a valid signal number. Alias expansion is always enabled. 32. it does not display them with a leading ‘alias ’ unless the ‘-p’ option is supplied. The trap builtin doesn’t check the first argument for a possible signal specification and revert the signal handling to the original disposition if it is. 26. When the cd builtin is invoked in logical mode. When the xpg_echo option is enabled. 39. instead of checking $VISUAL and $EDITOR. The type and command builtins will not report a non-executable file as having been found. 31. it does not display shell function names and definitions. and source builtins do not search the current directory for the filename argument if it is not found by searching PATH.
. Bash does not attempt to interpret any arguments to echo as options. 37. 27. The default editor used by fc is ed.

8 [Optional Features]. page 129). by specifying the ‘--enable-strict-posix-default’ to configure when building (see Section 10. Bash requires the xpg_echo option to be enabled for the echo builtin to be fully conformant. The fc builtin checks $EDITOR as a program to edit history entries if FCEDIT is unset.
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There is other posix behavior that Bash does not implement by default even when in posix mode. fc uses ed if EDITOR is unset. Specifically: 1. As noted above. Bash can be configured to be posix-conformant by default. rather than defaulting directly to ed. 2.

.

To facilitate the implementation of the user interface to job control. and has the additional side effect of causing pending output and typeahead to be discarded. Bash contains facilities to use it. The symbols ‘%%’ and ‘%+’ refer to the shell’s notion of the current job.1 Job Control Basics
Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. which may be listed with the jobs command. and how Bash allows you to access its facilities. unless caught. using the bg command to continue it in the background. When Bash starts a job asynchronously. A ‘^Z’ takes effect immediately. how it works. ‘%+’ and ‘%-’ can both be used to refer to that job.
. If the operating system on which Bash is running supports job control. These processes are said to be in the foreground. It keeps a table of currently executing jobs. There are a number of ways to refer to a job in the shell.. If there is only a single job. Control-Z) while a process is running causes that process to be stopped and returns control to Bash.g. All of the processes in a single pipeline are members of the same job. suspends the process. Bash uses the job abstraction as the basis for job control.
7. Typing the suspend character (typically ‘^Z’. Members of this process group (processes whose process group id is equal to the current terminal process group id) receive keyboard-generated signals such as SIGINT. The shell associates a job with each pipeline. Background processes which attempt to read from (write to when stty tostop is in effect) the terminal are sent a SIGTTIN (SIGTTOU) signal by the kernel’s terminal driver. Control-Y) causes the process to be stopped when it attempts to read input from the terminal. A user typically employs this facility via an interactive interface supplied jointly by the operating system kernel’s terminal driver and Bash. and control to be returned to Bash. In output pertaining to jobs (e. Job number n may be referred to as ‘%n’. the fg command to continue it in the foreground.Chapter 7: Job Control
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7 Job Control
This chapter discusses what job control is. Only foreground processes are allowed to read from or. The user then manipulates the state of this job. it prints a line that looks like: [1] 25647 indicating that this job is job number 1 and that the process id of the last process in the pipeline associated with this job is 25647. The character ‘%’ introduces a job specification (jobspec ). the operating system maintains the notion of a current terminal process group id. which. Background processes are those whose process group id differs from the terminal’s. such processes are immune to keyboard-generated signals. and the previous job with a ‘-’. if the user so specifies with stty tostop. the current job is always flagged with a ‘+’. A single ‘%’ (with no accompanying job specification) also refers to the current job. write to the terminal. or the kill command to kill it. The previous job may be referenced using ‘%-’. the output of the jobs command). which is the last job stopped while it was in the foreground or started in the background. Typing the delayed suspend character (typically ‘^Y’.

7. when run with job control enabled.. jobspec does not specify a valid job or jobspec specifies a job that was started without job control. lists the jobs and their statuses. or. (or running. or using a substring that appears in its command line. page 55). page 51). If a second attempt to exit is made without an intervening command. Bash waits until it is about to print a prompt before reporting changes in a job’s status so as to not interrupt any other output. Using ‘%?ce’. Normally. The return status is zero unless it is run when job control is not enabled. Similarly. Bash does not print another warning. the current job is used.3.2 Job Control Builtins
bg bg [jobspec . and if the checkjobs option is enabled.. on the other hand. and any stopped jobs are terminated. If the ‘-b’ option to the set builtin is enabled. when run with job control enabled. the current job is used. equivalent to ‘bg %1’ The shell learns immediately whenever a job changes state. The return status is that of the command placed into the foreground.3. ‘%ce’ refers to a stopped ce job. fg fg [jobspec] Resume the job jobspec in the foreground and make it the current job. Bash reports such changes immediately (see Section 4. or non-zero if run when job control is disabled or.2 [The Shopt Builtin]. If an attempt to exit Bash is made while jobs are stopped.] Resume each suspended job jobspec in the background. Any trap on SIGCHLD is executed for each child process that exits.1 [The Set Builtin]. If jobspec is not supplied. bringing job 1 from the background into the foreground. Bash reports an error. Display information only about jobs that have changed status since the user was last notified of their status. For example. Simply naming a job can be used to bring it into the foreground: ‘%1’ is a synonym for ‘fg %1’. The jobs command may then be used to inspect their status.
. If jobspec is not supplied. refers to any job containing the string ‘ce’ in its command line. jobs jobs [-lnprs] [jobspec] jobs -x command [arguments] The first form lists the active jobs. if the checkjobs option is enabled – see Section 4. ‘%1 &’ resumes job 1 in the background. The options have the following meanings: -l -n -p List process ids in addition to the normal information. the shell prints a warning message. as if it had been started with ‘&’. List only the process id of the job’s process group leader. If the prefix or substring matches more than one job.90
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A job may also be referred to using a prefix of the name used to start it. any jobspec was not found or specifies a job that was started without job control.

wait wait [jobspec or pid . The ‘-l’ option lists the signal names. the current job is used. all currently active child processes are waited for. but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP..] Wait until the child process specified by each process id pid or job specification jobspec exits and return the exit status of the last command waited for. kill kill [-s sigspec] [-n signum] [-sigspec] jobspec or pid kill -l [exit_status] Send a signal specified by sigspec or signum to the process named by job specification jobspec or process id pid. and executes command. If no arguments are given. If the ‘-x’ option is supplied. passing it arguments.] Without options. SIGTERM is used. and the return status is zero.. and the return status is zero. the return status is 127. If the ‘-h’ option is given. The return status is zero if at least one signal was successfully sent. the job is not removed from the table. If jobspec is not present..
. sigspec is either a case-insensitive signal name such as SIGINT (with or without the SIG prefix) or a signal number.
If jobspec is given. If jobspec is not supplied. output is restricted to information about that job. If neither jobspec nor pid specifies an active child process of the shell.. exit status is a number specifying a signal number or the exit status of a process terminated by a signal. If a job spec is given. suspend suspend [-f] Suspend the execution of this shell until it receives a SIGCONT signal. the ‘-a’ option means to remove or mark all jobs. signum is a signal number. If sigspec and signum are not present. or non-zero if an error occurs or an invalid option is encountered. returning its exit status. If any arguments are supplied when ‘-l’ is given. the status of all jobs is listed. the names of the signals corresponding to the arguments are listed. jobs replaces any jobspec found in command or arguments with the corresponding process group id. If no jobspec is supplied. all processes in the job are waited for. disown disown [-ar] [-h] [jobspec . the ‘-f’ option can be used to override this and force the suspension.Chapter 7: Job Control
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-r -s
Restrict output to running jobs. A login shell cannot be suspended. the ‘-r’ option without a jobspec argument restricts operation to running jobs. each jobspec is removed from the table of active jobs. and neither the ‘-a’ nor ‘-r’ option is supplied. Restrict output to stopped jobs.

.1 [Job Control Basics]. this provides functionality analogous to the ‘%’ job id. page 89). They must be supplied process ids. If this variable exists then single word simple commands without redirections are treated as candidates for resumption of an existing job. in this context. If this variable is set to the value ‘exact’. if there is more than one job beginning with the string typed. the kill and wait builtins do not accept jobspec arguments. if set to ‘substring’. the supplied string must be a prefix of a stopped job’s name. The name of a stopped job.3 Job Control Variables
auto_resume This variable controls how the shell interacts with the user and job control. the string supplied must match the name of a stopped job exactly. There is no ambiguity allowed. is the command line used to start it. then the most recently accessed job will be selected.92
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When job control is not active. the string supplied needs to match a substring of the name of a stopped job.
7. If set to any other value. The ‘substring’ value provides functionality analogous to the ‘%?’ job id (see Section 7.

and delete or insert the text of the corrections. DEL. The text M-k is read as ‘Meta-K’ and describes the character produced when the Meta key (if you have one) is depressed. you simply press RET. By default. If your keyboard lacks a LFD key. which is used by several different programs. such as a Compose key for typing accented characters. the identical keystroke can be generated by typing ESC first.Chapter 8: Command Line Editing
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8 Command Line Editing
This chapter describes the basic features of the gnu command line editing interface. page 51). the line editing commands are similar to those of emacs. page 41). The Readline library gives you a set of commands for manipulating the text as you type it in.3. page 96). or disabled using the ‘+o emacs’ or ‘+o vi’ options to set. and not forcing you to retype the majority of the line. the ALT on the left side is generally set to work as a Meta key. Either process is known as metafying the k key.
8. unless the ‘--noediting’ option is supplied at shell invocation. You do not have to be at the end of the line to press RET. Using these editing commands.
8. The Meta key is labeled ALT on many keyboards. On keyboards with two keys labeled ALT (usually to either side of the space bar). and TAB all stand for themselves when seen in this text. several keys have their own names. The ALT key on the right may also be configured to work as a Meta key or may be configured as some other modifier. typing C-j will produce the desired character. Command line editing is provided by the Readline library.1 [The Set Builtin]. If you do not have a Meta or ALT key. allowing you to just fix your typo. Command line editing is enabled by default when using an interactive shell. Line editing is also used when using the ‘-e’ option to the read builtin command (see Section 4. Specifically. the entire line is accepted regardless of the location of the cursor within the line. SPC. or another key working as a Meta key. ESC. Line editing can be enabled at any time using the ‘-o emacs’ or ‘-o vi’ options to the set builtin command (see Section 4. RET. LFD.2 Readline Interaction
Often during an interactive session you type in a long line of text. including Bash. when you are satisfied with the line.2 [Bash Builtins]. only to notice that the first word on the line is misspelled. and the k key is pressed. A vi-style line editing interface is also available. you move the cursor to the place that needs correction.3 [Readline Init File]. or in an init file (see Section 8.
. The RET key may be labeled Return or Enter on some keyboards. and then typing k. The text C-k is read as ‘Control-K’ and describes the character produced when the k key is pressed while the Control key is depressed. In addition.1 Introduction to Line Editing
The following paragraphs describe the notation used to represent keystrokes. Then. The text M-C-k is read as ‘Meta-Control-k’ and describes the character produced by metafying C-k.

Notice how C-f moves forward a character. reprinting the current line at the top. C-d Delete the character underneath the cursor. Clear the screen. In that case. C-a C-e M-f M-b C-l Move to the start of the line. The typed character appears where the cursor was.
Printing characters Insert the character into the line at the cursor. Move to the end of the line. C-f. and DEL. the Backspace key be set to delete the character to the left of the cursor and the DEL key set to delete the character underneath the cursor.2. and then the cursor moves one space to the right.94
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8. For your convenience. Afterwards. When you add text in the middle of a line.2. It is a loose convention that control keystrokes operate on characters while meta keystrokes operate on words. (Depending on your configuration. many other commands have been added in addition to C-b. Move backward a word. rather than the character to the left of the cursor. Move forward one character. C-d. you can move the cursor to the right with C-f. you can type C-b to move the cursor to the left. You can undo all the way back to an empty line. like C-d.1 Readline Bare Essentials
In order to enter characters into the line. simply type them. and not notice the error until you have typed several other characters. when you delete text behind the cursor.
DEL or Backspace Delete the character to the left of the cursor.2 Readline Movement Commands
The above table describes the most basic keystrokes that you need in order to do editing of the input line.
. If you mistype a character. characters to the right of the cursor are ‘pulled back’ to fill in the blank space created by the removal of the text. Move forward a word. A list of the bare essentials for editing the text of an input line follows. while M-f moves forward a word. you will notice that characters to the right of the cursor are ‘pushed over’ to make room for the text that you have inserted. you can use your erase character to back up and delete the mistyped character. and then correct your mistake. Likewise. C-b C-f Move back one character. C-_ or C-x C-u Undo the last editing command. Here are some commands for moving more rapidly about the line. where a word is composed of letters and digits. Sometimes you may mistype a character.)
8.

Here is the list of commands for killing text. Kill from the cursor the start of the current word. (‘Cut’ and ‘paste’ are more recent jargon for ‘kill’ and ‘yank’.5 Searching for Commands in the History
Readline provides commands for searching through the command history (see Section 9. to the end of the next word. you could type ‘M-1 0 C-d’.
C-w
Here is how to yank the text back into the line.4 Readline Arguments
You can pass numeric arguments to Readline commands. Word boundaries are the same as those used by M-b. to the start of the previous word. if between words. Any number of consecutive kills save all of the killed text together. the text is saved in a kill-ring.Chapter 8: Command Line Editing
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8.3 Readline Killing Commands
Killing text means to delete the text from the line. which will delete the next ten characters on the input line. If the first ‘digit’ typed is a minus sign (‘-’). page 121) for lines containing a specified string.1 [Bash History Facilities]. When you use a kill command. Rotate the kill-ring. For example. and then the command. then the sign of the argument will be negative.
. This is different than M-DEL because the word boundaries differ.2. and yank the new top. then you can be sure that you can get the text back in a different (or the same) place later.2. Kill from the cursor to the previous whitespace. other times it is the sign of the argument that is significant.2. For example. The kill ring is not line specific. or. C-y M-y Yank the most recently killed text back into the buffer at the cursor.
8. Sometimes the argument acts as a repeat count. to kill text back to the start of the line. Kill from the cursor to the end of the current word. or. you might type ‘M-. you can type the remainder of the digits.C-k’. If you pass a negative argument to a command which normally acts in a forward direction. but to save it away for later use. C-k M-d M-DEL Kill the text from the current cursor position to the end of the line. to give the C-d command an argument of 10. You can only do this if the prior command is C-y or M-y. if between words. Once you have typed one meta digit to get the argument started. There are two search modes: incremental and non-incremental. The general way to pass numeric arguments to a command is to type meta digits before the command. usually by yanking (re-inserting) it back into the line. when you are typing another line.
8. so that when you yank it back. the text that you killed on a previously typed line is available to be yanked back later. you get it all. that command will act in a backward direction.) If the description for a command says that it ‘kills’ text. Word boundaries are the same as those used by M-f. Yanking means to copy the mostrecently-killed text from the kill buffer.

An incremental search requires only as many characters as needed to find the desired history entry. To search backward in the history for a particular string. a RET will terminate the search and accept the line. Typing C-s searches forward through the history.3 Readline Init File
Although the Readline library comes with a set of Emacs-like keybindings installed by default.
8.
8.2 [Conditional Init Constructs]. thus incorporating any changes that you might have made to it. Blank lines are ignored. To find other matching entries in the history list. This will search backward or forward in the history for the next entry matching the search string typed so far. If that file does not exist or cannot be read. thereby executing the command from the history list. Readline displays the next entry from the history matching the string typed so far. A movement command will terminate the search. any remembered search string is used. conventionally in his home directory. Lines beginning with a ‘#’ are comments. Other lines denote variable settings and key bindings.3. If two C-rs are typed without any intervening characters defining a new search string. The syntax is simple:
. the ultimate default is ‘/etc/inputrc’. The name of this file is taken from the value of the shell variable INPUTRC. the C-x C-r command re-reads this init file. As each character of the search string is typed.3. Any other key sequence bound to a Readline command will terminate the search and execute that command. the history entry containing the search string becomes the current line. type C-r or C-s as appropriate. the init file is read. C-g will abort an incremental search and restore the original line. Variable Settings You can modify the run-time behavior of Readline by altering the values of variables in Readline using the set command within the init file. the default is ‘~/. Any user can customize programs that use Readline by putting commands in an inputrc file. The search string may be typed by the user or be part of the contents of the current line. and the key bindings are set. and begin editing. it is possible to use a different set of keybindings. Readline remembers the last incremental search string. For instance. When the search is terminated. make the last line found the current line. type C-r. the ESC and C-J characters will terminate an incremental search. When a program which uses the Readline library starts up. If that variable has not been assigned a value. Non-incremental searches read the entire search string before starting to search for matching history lines.96
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Incremental searches begin before the user has finished typing the search string. The characters present in the value of the isearch-terminators variable are used to terminate an incremental search. page 102).1 Readline Init File Syntax
There are only a few basic constructs allowed in the Readline init file.inputrc’. Lines beginning with a ‘$’ indicate conditional constructs (see Section 8. In addition. If that variable is unset.

The default value is ‘off’. common prefixes longer than this value are replaced with an ellipsis when displaying possible completions. Readline will ask the user whether or not he wishes to view them. This variable must be set to an integer value greater than or equal to 0. where appropriate.
. completion-query-items The number of possible completions that determines when the user is asked whether the list of possibilities should be displayed. The bind -V command lists the current Readline variable names and values. Any other value results in the variable being set to off. If set to ‘visible’. If set to ‘none’. are recognized without regard to case. If the number of possible completions is greater than this value. A great deal of run-time behavior is changeable with the following variables.Chapter 8: Command Line Editing
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set variable value Here. bind-tty-special-chars If set to ‘on’. The default limit is 100. Readline attempts to ring the terminal’s bell. page 41. Readline uses a visible bell if one is available. completion-ignore-case If set to ‘on’. comment-begin The string to insert at the beginning of the line when the insertcomment command is executed. See Section 4. they are simply listed. Unrecognized variable names are ignored. Readline attempts to bind the control characters treated specially by the kernel’s terminal driver to their Readline equivalents. If set to ‘audible’ (the default). is how to change from the default Emacs-like key binding to use vi line editing commands: set editing-mode vi Variable names and values. The default value is "#".2 [Bash Builtins]. Readline never rings the bell. for example. or 1. Readline performs filename matching and completion in a case-insensitive fashion. Boolean variables (those that can be set to on or off) are set to on if the value is null or empty. otherwise. A negative value means Readline should never ask. When set to a value greater than zero. bell-style Controls what happens when Readline wants to ring the terminal bell. completion-prefix-display-length The length in characters of the common prefix of a list of possible completions that is displayed without modification. on (case-insensitive).

echo-control-characters When set to ‘on’. the history code attempts to place the point (the current cursor position) at the same location on each history line retrieved with previous-history or next-history. The default value is ‘on’. Readline will try to enable the application keypad when it is called. Readline will convert characters with the eighth bit set to an ascii key sequence by stripping the eighth bit and prefixing an ESC character. This variable can be set to either ‘emacs’ or ‘vi’. Readline will try to enable any meta modifier key the terminal claims to support when it is called. Setting it to ‘on’ means that the text of the lines being edited will scroll horizontally on a single screen line when they are longer than the width of the
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convert-meta If set to ‘on’. Completion characters will be inserted into the line as if they had been mapped to self-insert. Readline will inhibit word completion. disable-completion If set to ‘On’. converting them to a meta-prefixed key sequence. The default is ‘on’. On many terminals. history-size Set the maximum number of history entries saved in the history list. where the keystrokes are most similar to Emacs. on operating systems that indicate they support it. readline echoes a character corresponding to a signal generated from the keyboard. The default is ‘off’. The default is ‘off’. If set to zero. Some systems need this to enable the arrow keys. Readline starts up in Emacs editing mode. the number of entries in the history list is not limited. expand-tilde If set to ‘on’. By default. enable-meta-key When set to ‘on’. The default is ‘off’. tilde expansion is performed when Readline attempts word completion. The default is ‘off’. enable-keypad When set to ‘on’. history-preserve-point If set to ‘on’. The default is ‘on’. the meta key is used to send eight-bit characters. editing-mode The editing-mode variable controls which default set of key bindings is used. horizontal-scroll-mode This variable can be set to either ‘on’ or ‘off’.

unless the leading ‘. page 95). causes Readline to display an asterisk (‘*’) at the start of history lines which have been modified. and vi-insert. This variable is ‘off’ by default. Readline will display characters with the eighth bit set directly rather than as a meta-prefixed escape sequence. The default is ‘on’. when set to ‘on’. emacs-ctlx. isearch-terminators The string of characters that should terminate an incremental search without subsequently executing the character as a command (see Section 8. This variable is ‘on’ by default. regardless of what the terminal claims it can support. this variable is set to ‘off’. output-meta If set to ‘on’.2. The value of the editing-mode variable also affects the default keymap. vi is equivalent to vi-command. The name meta-flag is a synonym for this variable. Acceptable keymap names are emacs.’ (hidden files) when performing filename completion. The default value is emacs.
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screen.5 [Searching]. match-hidden-files This variable. completed names which are symbolic links to directories have a slash appended (subject to the value of markdirectories). vi-command. causes Readline to match files whose names begin with a ‘. The default is ‘off’. emacs-meta. input-meta If set to ‘on’. completed directory names have a slash appended. mark-symlinked-directories If set to ‘on’. when set to ‘on’. vi. keymap Sets Readline’s idea of the current keymap for key binding commands. page-completions If set to ‘on’. This variable is ‘on’ by default. If this variable has not been given a value. vi-move. emacs-standard. instead of wrapping onto a new screen line. Readline uses an internal more-like pager to display a screenful of possible completions at a time. Readline will enable eight-bit input (it will not clear the eighth bit in the characters it reads). the characters ESC and C-J will terminate an incremental search. emacs is equivalent to emacs-standard. The default value is ‘off’. The default is ‘off’. By default.’ is supplied by the user in the filename to be completed. mark-modified-lines This variable.
mark-directories If set to ‘on’.

if this is enabled. If set to ‘on’. if any. The default value is ‘off’. words which have more than one possible completion cause the matches to be listed immediately instead of ringing the bell. For instance. First you need to find the name of the command that you want to change. history lines may be modified and retain individual undo lists across calls to readline. a character denoting a file’s type is appended to the filename when listing possible completions. Readline will undo all changes to history lines before returning when accept-line is executed. Readline will display completions with matches sorted horizontally in alphabetical order. skip-completed-text If set to ‘on’. and then the name of the command. readline does not insert characters from the completion that match characters after point in the word being completed. It’s only active when performing completion in the middle of a word. a colon. words which have more than one possible completion without any possible partial completion (the possible completions don’t share a common prefix) cause the matches to be listed immediately instead of ringing the bell. The default is ‘off’. The following sections contain tables of the command name. so portions of the word following the cursor are not duplicated. By default. attempting completion when the cursor is after the ‘e’ in ‘Makefile’ will result in ‘Makefile’ rather than ‘Makefilefile’. The name of
. revert-all-at-newline If set to ‘on’. this alters the default completion behavior when inserting a single match into the line. and a short description of what the command does. the default keybinding. simply place on a line in the init file the name of the key you wish to bind the command to. visible-stats If set to ‘on’.100
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print-completions-horizontally If set to ‘on’. The default is ‘off’. If enabled. If set to ‘on’. The default value is ‘off’. The default value is ‘off’. The default is ‘off’. show-all-if-ambiguous This alters the default behavior of the completion functions. Key Bindings The syntax for controlling key bindings in the init file is simple. There can be no space between the key name and the colon – that will be interpreted as part of the key name. show-all-if-unmodified This alters the default behavior of the completion functions in a fashion similar to show-all-if-ambiguous. Once you know the name of the command. assuming there is a single possible completion. rather than down the screen.

For example: Control-u: universal-argument Meta-Rubout: backward-kill-word Control-o: "> output" In the above example. depending on what you find most comfortable. M-DEL is bound to the function backward-kill-word. a second set of backslash escapes is available:
. RETURN. Some gnu Emacs style key escapes can be used. RUBOUT. page 41. SPC. LFD. to insert the text ‘> output’ into the line).Chapter 8: Command Line Editing
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the key can be expressed in different ways. "keyseq": function-name or macro keyseq differs from keyname above in that strings denoting an entire key sequence can be specified.2 [Bash Builtins]. In addition to command names. and ‘ESC [ 1 1 ~’ is bound to insert the text ‘Function Key 1’. but the special character names are not recognized. ESC. a single quote or apostrophe
In addition to the gnu Emacs style escape sequences. keyname : function-name or macro keyname is the name of a key spelled out in English. The bind -p command displays Readline function names and bindings in a format that can put directly into an initialization file. ESCAPE. as in the following example. The following gnu Emacs style escape sequences are available when specifying key sequences: \C\M\e \\ \" \’ control prefix meta prefix an escape character backslash ". C-u is bound to the function universalargument. readline allows keys to be bound to a string that is inserted when the key is pressed (a macro ). SPACE. "\C-u": universal-argument "\C-x\C-r": re-read-init-file "\e[11~": "Function Key 1" In the above example. See Section 4. A number of symbolic character names are recognized while processing this key binding syntax: DEL. ‘C-x C-r’ is bound to the function re-read-init-file. NEWLINE. C-u is again bound to the function universal-argument (just as it was in the first example). RET. a double quotation mark ’. by placing the key sequence in double quotes. and C-o is bound to run the macro expressed on the right hand side (that is. and TAB.

For example. the following binding will make ‘C-x \’ insert a single ‘\’ into the line: "\C-x\\": "\\"
8. or the application using Readline.2 Conditional Init Constructs
Readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor which allows key bindings and variable settings to be performed as the result of tests. no characters are required to isolate it. Unquoted text is assumed to be a function name. This may be used in conjunction with the ‘set keymap’ command. mode The mode= form of the $if directive is used to test whether Readline is in emacs or vi mode. to set bindings in the emacsstandard and emacs-ctlx keymaps only if Readline is starting out in emacs mode. $if The $if construct allows bindings to be made based on the editing mode. This allows sun to match both sun and sun-cmd. for instance. the backslash escapes described above are expanded. the terminal being used. The term= form may be used to include terminal-specific key bindings.102
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\a \b \d \f \n \r \t \v \nnn \xHH
alert (bell) backspace delete form feed newline carriage return horizontal tab vertical tab the eight-bit character whose value is the octal value nnn (one to three digits) the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
When entering the text of a macro. single or double quotes must be used to indicate a macro definition. Backslash will quote any other character in the macro text. The text of the test extends to the end of the line. In the macro body. The word on the right side of the ‘=’ is tested against both the full name of the terminal and the portion of the terminal name before the first ‘-’. for instance. There are four parser directives used.3. perhaps to bind the key sequences output by the terminal’s function keys.
term
application The application construct is used to include application-specific settings. including ‘"’ and ‘’’. Each program using the Readline library sets the application
.

as seen in the previous example. This could be used to bind key sequences to functions useful for a specific program. the following command adds a key sequence that quotes the current or previous word in Bash: $if Bash # Quote the current or previous word "\C-xq": "\eb\"\ef\"" $endif $endif $else $include This command.
. This directive takes a single filename as an argument and reads commands and bindings from that file.3 Sample Init File
Here is an example of an inputrc file. the following directive reads from ‘/etc/inputrc’: $include /etc/inputrc
8. and conditional syntax. For instance. For example. variable assignment. terminates an $if command. and you can test for a particular value.Chapter 8: Command Line Editing
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name.3. Commands in this branch of the $if directive are executed if the test fails. This illustrates key binding.

TAB: complete
This happens to be the default. ask the user if he wants to see all of them set completion-query-items 150
.
# Macros that are convenient for shell interaction $if Bash # edit the path "\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f" # prepare to type a quoted word -# insert open and close double quotes # and move to just after the open quote "\C-x\"": "\"\"\C-b" # insert a backslash (testing backslash escapes # in sequences and macros) "\C-x\\": "\\" # Quote the current or previous word "\C-xq": "\eb\"\ef\"" # Add a binding to refresh the line.Chapter 8: Command Line Editing
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#"\M-\C-[A": #"\M-\C-[B": C-q: quoted-insert $endif
previous-history next-history
# An old-style binding. which is unbound "\C-xr": redraw-current-line # Edit variable on current line. "\M-\C-v": "\C-a\C-k$\C-y\M-\C-e\C-a\C-y=" $endif # use a visible bell if one is available set bell-style visible # don’t strip characters to 7 bits when reading set input-meta on # allow iso-latin1 characters to be inserted rather # than converted to prefix-meta sequences set convert-meta off # display characters with the eighth bit set directly # rather than as meta-prefixed characters set output-meta on # if there are more than 150 possible completions for # a word.

Words are delimited by non-quoted shell metacharacters.1 Commands For Moving
beginning-of-line (C-a) Move to the start of the current line. backward-word (M-b) Move back to the start of the current or previous word. Words are composed of letters and digits.4 Bindable Readline Commands
This section describes Readline commands that may be bound to key sequences. this is unbound.": yank-last-arg $endif
8. Words are delimited by non-quoted shell metacharacters. shell-backward-word () Move back to the start of the current or previous word. for a more terse format. end-of-line (C-e) Move to the end of the line. By default. (See Section 4. suitable for an inputrc file. shell-forward-word () Move forward to the end of the next word. backward-char (C-b) Move back a character. In the following descriptions.
8. and mark refers to a cursor position saved by the set-mark command. clear-screen (C-l) Clear the screen and redraw the current line.2 [Bash Builtins]. forward-char (C-f) Move forward a character.) Command names without an accompanying key sequence are unbound by default. leaving the current line at the top of the screen. forward-word (M-f) Move forward to the end of the next word.4. page 41. Words are composed of letters and digits.106
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# For FTP $if Ftp "\C-xg": "get \M-?" "\C-xt": "put \M-?" "\M-. The text between the point and mark is referred to as the region. redraw-current-line () Refresh the current line. point refers to the current cursor position. You can list your key bindings by executing bind -P or. bind -p.
.

this command is unbound. This is a non-incremental search. This is an incremental search. this command is unbound. non-incremental-forward-search-history (M-n) Search forward starting at the current line and moving ‘down’ through the the history as necessary using a non-incremental search for a string supplied by the user. history-search-forward () Search forward through the history for the string of characters between the start of the current line and the point.2 Commands For Manipulating The History
accept-line (Newline or Return) Accept the line regardless of where the cursor is. By default. end-of-history (M->) Move to the end of the input history. This is an incremental search. With an argument n. history-search-backward () Search backward through the history for the string of characters between the start of the current line and the point. add it to the history list according to the setting of the HISTCONTROL and HISTIGNORE variables. next-history (C-n) Move ‘forward’ through the history list.4. If this line is non-empty.. insert the nth word from the previous command (the words in the previous command begin with word 0).Chapter 8: Command Line Editing
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8. fetching the previous command.e. previous-history (C-p) Move ‘back’ through the history list. the line currently being entered. i. forward-search-history (C-s) Search forward starting at the current line and moving ‘down’ through the the history as necessary. If this line is a modified history line. beginning-of-history (M-<) Move to the first line in the history. This is a non-incremental search. non-incremental-reverse-search-history (M-p) Search backward starting at the current line and moving ‘up’ through the history as necessary using a non-incremental search for a string supplied by the user. reverse-search-history (C-r) Search backward starting at the current line and moving ‘up’ through the history as necessary. then restore the history line to its original state. A negative argument inserts the nth word from the end of the previous
. fetching the next command. By default. yank-nth-arg (M-C-y) Insert the first argument to the previous command (usually the second word on the previous line) at point.

) Insert yourself.
8. behave exactly like yank-nth-arg. as if the ‘!$’ history expansion had been specified. for example. Negative arguments have no effect. then this transposes the last two characters of the line. or M-_) Insert last argument to the previous command (the last word of the previous history entry). quoted-insert (C-q or C-v) Add the next character typed to the line verbatim. The history expansion facilities are used to extract the last argument.108
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command. transpose-chars (C-t) Drag the character before the cursor forward over the character at the cursor. in which case the character behind the cursor is deleted. Once the argument n is computed. then return eof. A numeric argument means to kill the characters instead of deleting them. Successive calls to yank-last-arg move back through the history list..4. moving the cursor forward as well. but do not move the cursor. downcase-word (M-l) Lowercase the current (or following) word. transpose-words (M-t) Drag the word before point past the word after point. and the last character typed was not bound to delete-char. This is how to insert key sequences like C-q. self-insert (a. unless the cursor is at the end of the line. the argument is extracted as if the ‘!n’ history expansion had been specified. 1. With a negative argument. lowercase the previous word.
. If point is at the beginning of the line.. With a negative argument.3 Commands For Changing Text
delete-char (C-d) Delete the character at point. this is not bound to a key. If the insertion point is at the end of the line. . If the insertion point is at the end of the line. b. With an argument. forward-backward-delete-char () Delete the character under the cursor. By default. A. but do not move the cursor. there are no characters in the line. this transposes the last two words on the line. inserting the last argument of each line in turn. backward-delete-char (Rubout) Delete the character behind the cursor. uppercase the previous word. upcase-word (M-u) Uppercase the current (or following) word. yank-last-arg (M-. moving point past that word as well. !.

With an explicit positive numeric argument. Each call to readline() starts in insert mode. to the end of the next word. Word boundaries are the same as backward-word. switches to insert mode. With an explicit non-positive numeric argument. but do not move the cursor. kill-whole-line () Kill all characters on the current line.
. vi mode does overwrite differently. unix-filename-rubout () Kill the word behind point. switches to overwrite mode. Word boundaries are the same as shell-backwardword. this is unbound. unix-word-rubout (C-w) Kill the word behind point. delete-horizontal-space () Delete all spaces and tabs around point. unix-line-discard (C-u) Kill backward from the cursor to the beginning of the current line. shell-kill-word () Kill from point to the end of the current word. backward-kill-line (C-x Rubout) Kill backward to the beginning of the line. using white space and the slash character as the word boundaries. By default. to the end of the next word. In overwrite mode. kill-word (M-d) Kill from point to the end of the current word.4 Killing And Yanking
kill-line (C-k) Kill the text from point to the end of the line. this is unbound. By default.
8. This command affects only emacs mode. The killed text is saved on the kill-ring. overwrite-mode () Toggle overwrite mode. Word boundaries are the same as shell-forward-word. With a negative argument. no matter where point is. or if between words. Word boundaries are the same as forward-word.4. The killed text is saved on the kill-ring. this command is unbound. backward-kill-word () Kill the word behind point. using white space as a word boundary. Characters bound to backwarddelete-char replace the character before point with a space. backward-kill-word (M-DEL) Kill the word behind point. or if between words. By default. capitalize the previous word.Chapter 8: Command Line Editing
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capitalize-word (M-c) Capitalize the current (or following) word. characters bound to self-insert replace the text at point rather than pushing the text to the right.

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kill-region () Kill the text in the current region. By default, this command is unbound. copy-region-as-kill () Copy the text in the region to the kill buffer, so it can be yanked right away. By default, this command is unbound. copy-backward-word () Copy the word before point to the kill buffer. The word boundaries are the same as backward-word. By default, this command is unbound. copy-forward-word () Copy the word following point to the kill buffer. The word boundaries are the same as forward-word. By default, this command is unbound. yank (C-y) Yank the top of the kill ring into the buffer at point. yank-pop (M-y) Rotate the kill-ring, and yank the new top. You can only do this if the prior command is yank or yank-pop.

8.4.5 Specifying Numeric Arguments
digit-argument (M-0, M-1, ... M--) Add this digit to the argument already accumulating, or start a new argument. M-- starts a negative argument. universal-argument () This is another way to specify an argument. If this command is followed by one or more digits, optionally with a leading minus sign, those digits define the argument. If the command is followed by digits, executing universal-argument again ends the numeric argument, but is otherwise ignored. As a special case, if this command is immediately followed by a character that is neither a digit or minus sign, the argument count for the next command is multiplied by four. The argument count is initially one, so executing this function the first time makes the argument count four, a second time makes the argument count sixteen, and so on. By default, this is not bound to a key.

8.4.6 Letting Readline Type For You
complete (TAB) Attempt to perform completion on the text before point. The actual completion performed is application-specific. Bash attempts completion treating the text as a variable (if the text begins with ‘$’), username (if the text begins with ‘~’), hostname (if the text begins with ‘@’), or command (including aliases and functions) in turn. If none of these produces a match, filename completion is attempted. possible-completions (M-?) List the possible completions of the text before point.

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insert-completions (M-*) Insert all completions of the text before point that would have been generated by possible-completions. menu-complete () Similar to complete, but replaces the word to be completed with a single match from the list of possible completions. Repeated execution of menu-complete steps through the list of possible completions, inserting each match in turn. At the end of the list of completions, the bell is rung (subject to the setting of bell-style) and the original text is restored. An argument of n moves n positions forward in the list of matches; a negative argument may be used to move backward through the list. This command is intended to be bound to TAB, but is unbound by default. menu-complete-backward () Identical to menu-complete, but moves backward through the list of possible completions, as if menu-complete had been given a negative argument. delete-char-or-list () Deletes the character under the cursor if not at the beginning or end of the line (like delete-char). If at the end of the line, behaves identically to possiblecompletions. This command is unbound by default. complete-filename (M-/) Attempt filename completion on the text before point. possible-filename-completions (C-x /) List the possible completions of the text before point, treating it as a filename. complete-username (M-~) Attempt completion on the text before point, treating it as a username. possible-username-completions (C-x ~) List the possible completions of the text before point, treating it as a username. complete-variable (M-$) Attempt completion on the text before point, treating it as a shell variable. possible-variable-completions (C-x $) List the possible completions of the text before point, treating it as a shell variable. complete-hostname (M-@) Attempt completion on the text before point, treating it as a hostname. possible-hostname-completions (C-x @) List the possible completions of the text before point, treating it as a hostname. complete-command (M-!) Attempt completion on the text before point, treating it as a command name. Command completion attempts to match the text against aliases, reserved words, shell functions, shell builtins, and finally executable filenames, in that order.

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possible-command-completions (C-x !) List the possible completions of the text before point, treating it as a command name. dynamic-complete-history (M-TAB) Attempt completion on the text before point, comparing the text against lines from the history list for possible completion matches. dabbrev-expand () Attempt menu completion on the text before point, comparing the text against lines from the history list for possible completion matches. complete-into-braces (M-{) Perform filename completion and insert the list of possible completions enclosed within braces so the list is available to the shell (see Section 3.5.1 [Brace Expansion], page 18).

8.4.7 Keyboard Macros
start-kbd-macro (C-x () Begin saving the characters typed into the current keyboard macro. end-kbd-macro (C-x )) Stop saving the characters typed into the current keyboard macro and save the definition. call-last-kbd-macro (C-x e) Re-execute the last keyboard macro defined, by making the characters in the macro appear as if typed at the keyboard.

8.4.8 Some Miscellaneous Commands
re-read-init-file (C-x C-r) Read in the contents of the inputrc file, and incorporate any bindings or variable assignments found there. abort (C-g) Abort the current editing command and ring the terminal’s bell (subject to the setting of bell-style). do-uppercase-version (M-a, M-b, M-x, ...) If the metafied character x is lowercase, run the command that is bound to the corresponding uppercase character. prefix-meta (ESC) Metafy the next character typed. This is for keyboards without a meta key. Typing ‘ESC f’ is equivalent to typing M-f. undo (C-_ or C-x C-u) Incremental undo, separately remembered for each line. revert-line (M-r) Undo all changes made to this line. This is like executing the undo command enough times to get back to the beginning.

set-mark (C-@) Set the mark to the point. the output is formatted in such a way that it can be made part of an inputrc file. dump-variables () Print all of the settable variables and their values to the Readline output stream. the output is formatted in such a way that it can be made part of an inputrc file. the mark is set to that position. the value is inserted. but usually bound to ESC-[. A negative count searches for subsequent occurrences. keys producing such sequences will have no effect unless explicitly bound to a readline command. If a numeric argument causes the comment character to be removed. the output is formatted in such a way that it can be made part of an inputrc file. instead of inserting stray characters into the editing buffer. This command is unbound by default. If a numeric argument is supplied. If a numeric argument is supplied. If a numeric argument is supplied.Chapter 8: Command Line Editing
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tilde-expand (M-&) Perform tilde expansion on the current word. If a numeric argument is supplied. dump-macros () Print all of the Readline key sequences bound to macros and the strings they output. usually ESC-[. This command is unbound by default. If a numeric argument is supplied.
. this command acts as a toggle: if the characters at the beginning of the line do not match the value of comment-begin. The current cursor position is set to the saved position. In either case. character-search-backward (M-C-]) A character is read and point is moved to the previous occurrence of that character. the line is accepted as if a newline had been typed. dump-functions () Print all of the functions and their key bindings to the Readline output stream. This command is unbound by default. If this sequence is bound to "\e[". the value of the comment-begin variable is inserted at the beginning of the current line. This is unbound by default. skip-csi-sequence () Read enough characters to consume a multi-key sequence such as those defined for keys like Home and End. character-search (C-]) A character is read and point is moved to the next occurrence of that character. A negative count searches for previous occurrences. exchange-point-and-mark (C-x C-x) Swap the point with the mark. insert-comment (M-#) Without a numeric argument. otherwise the characters in comment-begin are deleted from the beginning of the line. Such sequences begin with a Control Sequence Indicator (CSI). and the old cursor position is saved as the mark. The default value of comment-begin causes this command to make the current line a shell comment. the line will be executed by the shell.

page 123). and the list of matching file names is inserted.
8. This performs alias and history expansion as well as all of the shell word expansions (see Section 3.114
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glob-complete-word (M-g) The word before point is treated as a pattern for pathname expansion. history-expand-line (M-^) Perform history expansion on the current line. insert-last-argument (M-. edit-and-execute-command (C-xC-e) Invoke an editor on the current command line.2 standard. or M-_) A synonym for yank-last-arg. history-and-alias-expand-line () Perform history and alias expansion on the current line. This pattern is used to generate a list of matching file names for possible completions.6 [Aliases]. with an asterisk implicitly appended. a ‘*’ is appended before pathname expansion. The Readline vi mode behaves as specified in the posix 1003. glob-expand-word (C-x *) The word before point is treated as a pattern for pathname expansion. magic-space () Perform history expansion on the current line and insert a space (see Section 9.3 [History Interaction]. page 79). glob-list-expansions (C-x g) The list of expansions that would have been generated by glob-expand-word is displayed. page 17).5 Readline vi Mode
While the Readline library does not have a full set of vi editing functions. operate-and-get-next (C-o) Accept the current line for execution and fetch the next line relative to the current line from the history for editing. Any argument is ignored. replacing the word. $EDITOR. a ‘*’ is appended before pathname expansion. display-shell-version (C-x C-v) Display version information about the current instance of Bash. in that order. and emacs as the editor. Bash attempts to invoke $VISUAL. If a numeric argument is supplied.5 [Shell Expansions]. and the line is redrawn. If a numeric argument is supplied. it does contain enough to allow simple editing of the line.
. shell-expand-line (M-C-e) Expand the line as the shell does. and execute the result as shell commands. alias-expand-line () Perform alias expansion on the current line (see Section 6.

The GLOBIGNORE shell variable is not used to filter the matches. as described above (see Section 3. When the command or function is invoked. If the command word is the empty string (completion attempted at the beginning of an empty line). If a compspec has been defined for that command. The results are split using the rules described above (see Section 3. any compspec defined with the ‘-E’ option to complete is used. and COMP_TYPE variables are assigned values as described above (see Section 5. page 110) is performed. Pressing ESC switches you into ‘command’ mode. any shell function or command specified with the ‘-F’ and ‘-C’ options is invoked. the COMP_ LINE. the string specified as the argument to the ‘-W’ option is considered. command substitution. When the ‘-f’ or ‘-d’ option is used for filename or directory name completion. If those searches do not result in a compspec.2 [Bash Variables]. an attempt is made to find a compspec for the portion following the final slash. page 51). COMP_POINT. First.7 [Programmable Completion Builtins]. The results of the expansion are prefix-matched against the word being completed. for a description of FIGNORE. parameter and variable expansion.5 [Shell Expansions]. the command name is identified. use the ‘set -o emacs’ and ‘set -o vi’ commands (see Section 4. Only matches which are prefixed by the word being completed are returned. but the FIGNORE shell variable is used. page 61).7 [Word Splitting]. page 17). The Readline default is emacs mode. tilde expansion. any compspec defined with the ‘-D’ option to complete is used as the default. and so forth. See Section 5. and the matching words become the possible completions. If the command word is a full pathname.
8. Shell quoting is honored. Any completions specified by a filename expansion pattern to the ‘-G’ option are generated next. First. The words generated by the pattern need not match the word being completed. the compspec is used to generate the list of possible completions for the word. COMP_KEY. it is used to generate the list of matching words. the shell variable FIGNORE is used to filter the matches.4. page 61. If a shell function is being invoked. If a compspec is not found. the programmable completion facilities are invoked. page 23). you are already placed in ‘insertion’ mode.3.6 [Commands For Completion]. page 117). and arithmetic expansion.6 Programmable Completion
When word completion is attempted for an argument to a command for which a completion specification (a compspec ) has been defined using the complete builtin (see Section 8. the actions specified by the compspec are used.Chapter 8: Command Line Editing
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In order to switch interactively between emacs and vi editing modes. When the function or command
. The string is first split using the characters in the IFS special variable as delimiters. the default Bash completion described above (see Section 8.2 [Bash Variables].1 [The Set Builtin]. where you can edit the text of the line with the standard vi movement keys. a compspec for the full pathname is searched for first. as if you had typed an ‘i’.5. Once a compspec has been found. the COMP_WORDS and COMP_CWORD variables are also set. When you enter a line in vi mode. move to previous history lines with ‘k’ and subsequent lines with ‘j’. Next. If no compspec is found for the full pathname. Each word is then expanded using brace expansion. After these matches have been generated.

subject to the value of the mark-directories Readline variable. A literal ‘&’ may be escaped with a backslash. any filter specified with the ‘-X’ option is applied to the list. If the previously-applied actions do not generate any matches. The function may use any of the shell facilities. if a compspec is found.116
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is invoked. directory name completion is attempted. The filter is a pattern as used for pathname expansion. It should print a list of completions. if necessary. If the ‘-o default’ option was supplied to complete when the compspec was defined. the backslash is removed before attempting a match. No filtering of the generated completions against the word being completed is performed. The default Bash completions are not attempted. the default Bash completions) generate no matches. Any completion that matches the pattern will be removed from the list. and the third argument is the word preceding the word being completed on the current command line. one per line. the second argument is the word being completed. If the ‘-o plusdirs’ option was supplied to complete when the compspec was defined. page 117). Backslash may be used to escape a newline. A leading ‘!’ negates the pattern. It must put the possible completions in the COMPREPLY array variable. if attempted. any command specified with the ‘-C’ option is invoked in an environment equivalent to command substitution. to generate the matches. in this case any completion not matching the pattern will be removed. By default. When a compspec indicates that directory name completion is desired. If the ‘-o bashdefault’ option was supplied to complete when the compspec was defined. and changes the compspec associated with the command on which completion is being attempted (supplied as the first argument when the function is executed). After all of the possible completions are generated. whatever it generates is returned to the completion code as the full set of possible completions. Readline’s default completion will be performed if the compspec (and. directory name completion is attempted and any matches are added to the results of the other actions. Finally. the function or command has complete freedom in generating the matches.7 [Programmable Completion Builtins]. to the standard output. with an attempt to find a compspec for that command. and the Readline default of filename completion is disabled. the first argument is the name of the command whose arguments are being completed. regardless of the setting of the mark-symlinked-directories Readline variable. Next. the programmable completion functions force Readline to append a slash to completed names which are symbolic links to directories. If a shell function returns 124. and the result is returned to the Readline completion code as the list of possible completions. and the ‘-o dirnames’ option was supplied to complete when the compspec was defined. Any function specified with ‘-F’ is invoked first. including the compgen and compopt builtins described below (see Section 8. the default Bash completions are attempted if the compspec generates no matches. any prefix and suffix specified with the ‘-P’ and ‘-S’ options are added to each member of the completion list. This allows a set of
. This is most useful when used in combination with a default completion specified with ‘-D’. It’s possible for shell functions executed as completion handlers to indicate that completion should be retried by returning an exit status of 124. a ‘&’ in the pattern is replaced with the text of the word being completed. There is some support for dynamically modifying completions. programmable completion restarts from the beginning.

. which may be any option accepted by the complete builtin with the exception of ‘-p’ and ‘-r’. For instance. will not have useful values. The matches will be generated in the same way as if the programmable completion code had generated them directly from a completion specification with the same flags. or..sh" >/dev/null 2>&1 && return 124 } complete -D -F _completion_loader
8. The ‘-D’ option indicates that the remaining options and actions should apply to the “default” command completion. and write the matches to the standard output. If the ‘-p’ option is supplied. that is. while available. compgen compgen [option] [word] Generate possible completion matches for word according to the options. or if no options are supplied.. only those completions matching word will be displayed. "/etc/bash_completion. rather than being loaded all at once.] complete -pr [-DE] [name . The ‘-E’ option indicates that the remaining options and actions should apply to “empty” command completion..7 Programmable Completion Builtins
Two builtin commands are available to manipulate the programmable completion facilities. if no name s are supplied. complete complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action] [G globpat] [-W wordlist] [-F function] [-C command] [-X filterpat] [-P prefix] [-S suffix] name [name . If word is specified.] Specify how arguments to each name should be completed.d/$1. that is. The return value is true unless an invalid option is supplied. or no matches were generated.Chapter 8: Command Line Editing
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completions to be built dynamically as completion is attempted. When using the ‘-F’ or ‘-C’ options. assuming that there is a library of compspecs. the following default completion function would load completions dynamically: _completion_loader() { . existing completion specifications are printed in a way that allows them to be reused as input. all completion specifications. completion attempted on a blank line. completion attempted on a command for which no completion has previously been defined. each kept in a file corresponding to the name of the command.. the various shell variables set by the programmable completion facilities. The ‘-r’ option removes a completion specification for each name.

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The process of applying these completion specifications when word completion is attempted is described above (see Section 8.6 [Programmable Completion], page 115). The ‘-D’ option takes precedence over ‘-E’. Other options, if specified, have the following meanings. The arguments to the ‘-G’, ‘-W’, and ‘-X’ options (and, if necessary, the ‘-P’ and ‘-S’ options) should be quoted to protect them from expansion before the complete builtin is invoked. -o comp-option The comp-option controls several aspects of the compspec’s behavior beyond the simple generation of completions. comp-option may be one of: bashdefault Perform the rest of the default Bash completions if the compspec generates no matches. default dirnames filenames Tell Readline that the compspec generates filenames, so it can perform any filename-specific processing (like adding a slash to directory names quoting special characters, or suppressing trailing spaces). This option is intended to be used with shell functions specified with ‘-F’. nospace plusdirs Tell Readline not to append a space (the default) to words completed at the end of the line. After any matches defined by the compspec are generated, directory name completion is attempted and any matches are added to the results of the other actions. Use Readline’s default filename completion if the compspec generates no matches. Perform directory name completion if the compspec generates no matches.

-A action The action may be one of the following to generate a list of possible completions: alias arrayvar binding builtin command directory Directory names. May also be specified as ‘-d’. Alias names. May also be specified as ‘-a’. Array variable names. Readline key binding names (see Section 8.4 [Bindable Readline Commands], page 106). Names of shell builtin commands. May also be specified as ‘-b’. Command names. May also be specified as ‘-c’.

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disabled enabled export file function group helptopic

Names of disabled shell builtins. Names of enabled shell builtins. Names of exported shell variables. May also be specified as ‘-e’. File names. May also be specified as ‘-f’. Names of shell functions. Group names. May also be specified as ‘-g’. Help topics as accepted by the help builtin (see Section 4.2 [Bash Builtins], page 41).

hostname

Hostnames, as taken from the file specified by the HOSTFILE shell variable (see Section 5.2 [Bash Variables], page 61). Job names, if job control is active. May also be specified as ‘-j’. Shell reserved words. May also be specified as ‘-k’. Names of running jobs, if job control is active. Service names. May also be specified as ‘-s’. Valid arguments for the ‘-o’ option to the set builtin (see Section 4.3.1 [The Set Builtin], page 51). Shell option names as accepted by the shopt builtin (see Section 4.2 [Bash Builtins], page 41). Signal names. Names of stopped jobs, if job control is active. User names. May also be specified as ‘-u’. Names of all shell variables. May also be specified as ‘-v’.

The filename expansion pattern globpat is expanded to generate the possible completions. -W wordlist The wordlist is split using the characters in the IFS special variable as delimiters, and each resultant word is expanded. The possible completions are the members of the resultant list which match the word being completed. -C command command is executed in a subshell environment, and its output is used as the possible completions.

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-F function The shell function function is executed in the current shell environment. When it finishes, the possible completions are retrieved from the value of the COMPREPLY array variable. -X filterpat filterpat is a pattern as used for filename expansion. It is applied to the list of possible completions generated by the preceding options and arguments, and each completion matching filterpat is removed from the list. A leading ‘!’ in filterpat negates the pattern; in this case, any completion not matching filterpat is removed. -P prefix prefix is added at the beginning of each possible completion after all other options have been applied. -S suffix suffix is appended to each possible completion after all other options have been applied. The return value is true unless an invalid option is supplied, an option other than ‘-p’ or ‘-r’ is supplied without a name argument, an attempt is made to remove a completion specification for a name for which no specification exists, or an error occurs adding a completion specification. compopt compopt [-o option] [-DE] [+o option] [name] Modify completion options for each name according to the options, or for the currently-execution completion if no name s are supplied. If no options are given, display the completion options for each name or the current completion. The possible values of option are those valid for the complete builtin described above. The ‘-D’ option indicates that the remaining options should apply to the “default” command completion; that is, completion attempted on a command for which no completion has previously been defined. The ‘-E’ option indicates that the remaining options should apply to “empty” command completion; that is, completion attempted on a blank line. The ‘-D’ option takes precedence over ‘-E’. The return value is true unless an invalid option is supplied, an attempt is made to modify the options for a name for which no completion specification exists, or an output error occurs.

The text of the last $HISTSIZE commands (default 500) is saved. The file named by the value of HISTFILE is truncated. the time stamp information associated with each history entry is written to the history file.2 [Commands For History]. to contain no more than the number of lines specified by the value of the HISTFILESIZE variable. The lithist shell option causes the shell to save the command with embedded newlines instead of semicolons. page 41. The value of the HISTSIZE shell variable is used as the number of commands to save in a history list. adding semicolons where necessary to preserve syntactic correctness. for a description of shopt. the list of commands previously typed. the history file is truncated to contain no more than $HISTFILESIZE lines. The builtin command fc may be used to list or edit and re-execute a portion of the history list. See Section 4. The shell stores each command in the history list prior to parameter and variable expansion but after history expansion is performed. search commands are available in each editing mode that provide access to the history list (see Section 8.1 Bash History Facilities
When the ‘-o history’ option to the set builtin is enabled (see Section 4. the lines are appended to the history file. lines beginning with the history comment character followed immediately by a digit are interpreted as timestamps for the previous history line. The shell allows control over which commands are saved on the history list.Chapter 9: Using History Interactively
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9 Using History Interactively
This chapter describes how to use the gnu History Library interactively. If the histappend shell option is set (see Section 4.3.1 [The Set Builtin].2 Bash History Builtins
Bash provides two builtin commands which manipulate the history list and history file. causes the shell to attempt to save each line of a multi-line command in the same history entry. The cmdhist shell option. the history is not saved. If HISTFILE is unset. if necessary.
9. The history builtin may be used to display or modify the history list and manipulate the history file. the history is initialized from the file named by the HISTFILE variable (default ‘~/. subject to the values of the shell variables HISTIGNORE and HISTCONTROL. fc
. It should be considered a user’s guide.bash_history’). When an interactive shell exits. When using command-line editing. or if the history file is unwritable. page 51). the last $HISTSIZE lines are copied from the history list to the file named by $HISTFILE. After saving the history. from a user’s standpoint.2 [Bash Builtins]. page 41). see the gnu Readline Library Manual. For information on using the gnu History Library in other programs. If the HISTTIMEFORMAT is set. no truncation is performed. If HISTFILESIZE is not set. the shell provides access to the command history.2 [Bash Builtins]. The shopt builtin is used to set these options. When the shell starts up. The HISTCONTROL and HISTIGNORE variables may be set to cause the shell to save only a subset of the commands entered. page 107). if enabled. marked with the history comment character.
9.4. When the history file is read. otherwise the history file is overwritten.

Otherwise.
-d offset Delete the history entry at position offset. If the ‘-l’ flag is given. In the second form. This may be combined with the other options to replace the history list completely. a range of commands from first to last is selected from the history list. history history history history history history [n] -c -d offset [-anrw] [filename] -ps arg
With no options. the editor given by ename is invoked on a file containing those commands.
. If first is not specified it is set to the previous command for editing and −16 for listing. This says to use the value of the FCEDIT variable if set. No intervening blank is printed between the formatted time stamp and the history line. if supplied. display the history list with line numbers. If last is not specified it is set to first. or the value of the EDITOR variable if that is set. An argument of n lists only the last n lines. These are lines appended to the history file since the beginning of the current Bash session. Both first and last may be specified as a string (to locate the most recent command beginning with that string) or as a number (an index into the history list. so that typing ‘r cc’ runs the last command beginning with cc and typing ‘r’ re-executes the last command (see Section 6. the commands are listed on standard output.6 [Aliases]. have the following meanings: -c Clear the history list. Options.122
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fc [-e ename] [-lnr] [first] [last] fc -s [pat=rep] [command] Fix Command. the edited commands are echoed and executed. A useful alias to use with the fc command is r=’fc -s’. command is re-executed after each instance of pat in the selected command is replaced by rep. The ‘-r’ flag reverses the order of the listing. Append the history lines not already read from the history file to the current history list. where a negative number is used as an offset from the current command number). The ‘-n’ flag suppresses the command numbers when listing. -a -n Append the new history lines (history lines entered since the beginning of the current Bash session) to the history file. In the first form. If the shell variable HISTTIMEFORMAT is set and not null. it is used as a format string for strftime to display the time stamp associated with each displayed history entry. If ename is not given. page 79). offset should be specified as it appears when the history is displayed. Lines prefixed with a ‘*’ have been modified. the value of the following variable expansion is used: ${FCEDIT:-${EDITOR:-vi}}. When editing is complete. or vi if neither is set.

‘-r’. If not. History expansion takes place in two parts. If Readline is being used.2 [Bash Variables]. without storing the results in the history list. then it is used as the history file. This is most useful in conjunction with Readline. ‘-a’. If the histverify shell option is enabled. The line is broken into words in the same fashion that Bash does.
9. making it easy to repeat commands. Instead. The shell uses the history comment character to mark history timestamps when writing the history file.
When any of the ‘-w’. history substitutions are not immediately passed to the shell parser. Several shell options settable with the shopt builtin (see Section 4. then the value of the HISTFILE variable is used. Various modifiers are available to manipulate the selected words.3 History Expansion
The History library provides a history expansion feature that is similar to the history expansion provided by csh. This section describes the syntax used to manipulate the history information. so that several words surrounded by quotes are considered one word. or fix errors in previous commands quickly. if filename is given. The shell allows control of the various characters used by the history expansion mechanism with the histchars variable. a failed history expansion will be reloaded into the Readline editing buffer for correction. page 61). The first is to determine which line from the history list should be used during substitution. Only ‘\’ and ‘’’ may be used to escape the history expansion character. or ‘-n’ options is used. and the histreedit shell option is enabled.Chapter 9: Using History Interactively
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-r -w -p -s
Read the current history file and append its contents to the history list.
9. and the portions of that line that are acted upon are called words. and Readline is being used. The ‘-s’ option to the history builtin may be used to add commands to the end of the history list without actually executing them.3. as explained above (see Section 5.1 Event Designators
An event designator is a reference to a command line entry in the history list. The second is to select portions of that line for inclusion into the current one.
. the expanded line is reloaded into the Readline editing buffer for further modification. History expansions are introduced by the appearance of the history expansion character. The ‘-p’ option to the history builtin command may be used to see what a history expansion will do before using it. Perform history substitution on the arg s and display the result on the standard output. insert the arguments to a previous command into the current input line.2 [Bash Builtins]. which is ‘!’ by default. The arg s are added to the end of the history list as a single entry. History expansions introduce words from the history list into the input stream. so that they are available for subsequent recall. page 41) may be used to tailor the behavior of history expansion. Write out the current history to the history file. The line selected from the history is called the event.

When you type this. tab. Words are numbered from the beginning of the line. this is the command word. Equivalent to !!:s/string1/string2/.
. This is a synonym for ‘!-1’. word 1. The nth word. For many applications. ‘-’. Refer to the command n lines back. !# The entire command line typed so far. A range of words. replacing string1 with string2. Refer to command line n. except when followed by a space.3. or ‘%’. A ‘:’ separates the event specification from the word designator. The last argument. the preceding command is repeated in toto. Words are inserted into the current line separated by single spaces. The first argument. with the first word being denoted by 0 (zero). the end of the line. Refer to the most recent command starting with string.
9. that is.2 Word Designators
Word designators are used to select desired words from the event. This may be shortened to !$.
!n !-n !! !string
!?string[?] Refer to the most recent command containing string. designates the last argument of the preceding command. This is a synonym for ‘1-$’. Repeat the last command. ‘=’ or ‘(’ (when the extglob shell option is enabled using the shopt builtin). designates the second argument of the most recent command starting with the letters fi. The trailing ‘?’ may be omitted if the string is followed immediately by a newline. All of the words.
Here are the word designators: 0 (zero) n ^ $ % x-y * The 0th word. ^string1^string2^ Quick Substitution. The word matched by the most recent ‘?string?’ search. ‘*’. It is not an error to use ‘*’ if there is just one word in the event. It may be omitted if the word designator begins with a ‘^’. For example. !! !!:$ !fi:2 designates the preceding command. the empty string is returned in that case. except the 0th. Refer to the previous command. ‘-y’ abbreviates ‘0-y’. ‘$’.124
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!
Start a history substitution.

leaving the basename.3 Modifiers
After the optional word designator.3. or with ‘&’. as in gs/old/new/. but break into words at spaces. If ‘&’ appears in new.
If a word designator is supplied without an event specification. but omits the last word. The final delimiter is optional if it is the last character on the input line.
9. Quote the substituted words as with ‘q’. each preceded by a ‘:’. Remove all leading pathname components. Any delimiter may be used in place of ‘/’.Chapter 9: Using History Interactively
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x* x-
Abbreviates ‘x-$’ Abbreviates ‘x-$’ like ‘x*’. the previous command is used as the event. Quote the substituted words. h t r e p q x s/old/new/ Substitute new for the first occurrence of old in the event line. Print the new command but do not execute it. Remove a trailing pathname component. Remove all but the trailing suffix. A single backslash will quote the ‘&’. The delimiter may be quoted in old and new with a single backslash. and newlines. leaving only the head. it is replaced by old. tabs. & g a G Repeat the previous substitution. Used in conjunction with ‘s’. Remove a trailing suffix of the form ‘. Cause changes to be applied over the entire event line.suffix’. Apply the following ‘s’ modifier once to each word in the event. escaping further substitutions.
. leaving the tail. you can add a sequence of one or more of the following modifiers.

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and several others).org so they can be considered for the next release. and several non-Unix systems such as BeOS and Interix. 2. The distribution supports the gnu operating systems. If you’re using csh on an old version of System V./configure --help at the Bash prompt in your Bash source directory. cd to the directory containing the source code and type ‘.
10. Finally. and ‘support’ directories. To also remove the files that configure created (so you can compile Bash for a different kind of computer). 4. While running.1 Basic Installation
These are installation instructions for Bash. If at some point ‘config. os/2. Running configure takes some time.04$ . The simplest way to compile Bash is: 1. it creates a shell script named config. type ‘make distclean’. please try to figure out how configure could check whether or not to do them.
. and mail diffs or instructions to bash-maintainers@gnu.Chapter 10: Installing Bash
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10 Installing Bash
This chapter provides basic instructions for installing Bash on the various supported platforms. Other independent ports exist for ms-dos. type ‘make tests’ to run the Bash test suite. You only need ‘configure. To find out more about the options and arguments that the configure script understands. it prints messages telling which features it is checking for. The configure shell script attempts to guess correct values for various system-dependent variables used during compilation. The file ‘configure.in’ is used to create configure by a program called Autoconf. 3./configure’ to configure Bash for your system. and a file ‘config. ‘doc’. make sure you are using Autoconf version 2.h’ file containing system-dependent definitions.50 or newer. This will also install the manual pages and Info file. you might need to type ‘sh .in’ if you want to change it or regenerate configure using a newer version of Autoconf.cache’ that saves the results of its tests to speed up reconfiguring. Type ‘make install’ to install bash and bashbug. If you do this. a file ‘config./configure’ instead to prevent csh from trying to execute configure itself. each directory under ‘lib’. nearly every version of Unix. type bash-2. It uses those values to create a ‘Makefile’ in each directory of the package (the top directory. you may remove or edit it. and Windows platforms. If you need to do unusual things to compile Bash.status that you can run in the future to recreate the current configuration. Type ‘make’ to compile Bash and build the bashbug bug reporting script.log’ containing compiler output (useful mainly for debugging configure). the ‘builtins’. Optionally. You can remove the program binaries and object files from the source code directory by typing ‘make clean’. It also creates a ‘config.cache’ contains results you don’t want to keep.

give it the ‘--host=TYPE’
.0’: bash /usr/gnu/src/bash-2. You can specify an installation prefix other than ‘/usr/local’ by giving configure the option ‘--prefix=PATH’. or by specifying a value for the DESTDIR ‘make’ variable when running ‘make install’. You may need to supply the ‘--srcdir=PATH’ argument to tell configure where the source files are. etc./configure The configuration process uses GCC to build Bash if it is available. you can compile Bash for one architecture at a time in the source code directory.3 Compiling For Multiple Architectures
You can compile Bash for more than one kind of computer at the same time.
10. configure automatically checks for the source code in the directory that configure is in and in ‘. Alternatively. by placing the object files for each architecture in their own directory. you can do that on the command line like this: CC=c89 CFLAGS=-O2 LIBS=-lposix . Using a Bourne-compatible shell. but need to determine by the type of host Bash will run on.128
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10.
10.. Here’s an example that creates a build directory in the current directory from a source directory ‘/usr/gnu/src/bash-2. To do this. You can give configure initial values for variables by setting them in the environment. such as GNU make. you must use a version of make that supports the VPATH variable. Documentation and other data files will still use the regular prefix. ‘make install’ will install into ‘/usr/local/bin’. so you must have already built Bash for at least one architecture before you can create build directories for other architectures. If you give configure the option ‘--exec-prefix=PATH’. you can do it like this: env CPPFLAGS=-I/usr/local/include LDFLAGS=-s . but if it prints a message saying it can not guess the host type.2 Compilers and Options
Some systems require unusual options for compilation or linking that the configure script does not know about.4 Installation Names
By default. ‘/usr/local/man’. If you have to use a make that does not supports the VPATH variable./configure On systems that have the env program. ‘make install’ will use PATH as the prefix for installing programs and libraries. use ‘make distclean’ before reconfiguring for another architecture.0/support/mkclone -s /usr/gnu/src/bash-2. you can use the ‘support/mkclone’ script to create a build tree which has symbolic links back to each file in the source directory. The mkclone script requires Bash. After you have installed Bash for one architecture. You can specify separate installation prefixes for architecture-specific files and architecture-independent files. cd to the directory where you want the object files and executables to go and run the configure script from the source directory.0 .
10.’. Usually configure can figure that out.5 Specifying the System Type
There may be some features configure can not figure out automatically. if your system supports symbolic links.

Do not print messages saying which checks are being made. and prefix./config.g. where package is something like ‘bash-malloc’ or ‘purify’. To configure Bash without a feature that is enabled by default.
--srcdir=dir Look for the Bash source code in directory dir.7 Operation Controls
configure recognizes the following options to control how it operates. To turn off the default use of a package. for debugging configure. boilerplate options. but not all configure scripts do. you can set the CONFIG_SITE environment variable to the location of the site script. Or.site that gives default values for variables like CC. Set file to ‘/dev/null’ to disable caching. This is not the same malloc that appears in gnu libc. such as ‘sun4’. configure looks for ‘PREFIX/share/config.
10.cache’. configure also accepts some other. ‘TYPE’ can either be a short name for the system type. See the file ‘support/config. --cache-file=file Use and save the results of the tests in file instead of ‘. and exit. then ‘PREFIX/etc/config. use ‘--without-package’. cache_ file. A warning: the Bash configure looks for a site script.sub’ for the possible values of each field. There are also several ‘--with-package’ options.8 Optional Features
The Bash configure has a number of ‘--enable-feature’ options.2’). Here is a complete list of the ‘--enable-’ and ‘--with-’ options that the Bash configure recognizes. and exit. ‘i386-unknown-freebsd4. --with-afs Define if you are using the Andrew File System from Transarc. Usually configure can determine that directory automatically.site’ if it exists. --help --quiet --silent -q Print a summary of the options to configure. not widely used. --version Print the version of Autoconf used to generate the configure script.site’ if it exists. but an older version originally derived
.6 Sharing Defaults
If you want to set default values for configure scripts to share.Chapter 10: Installing Bash
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option.. or a canonical name with three fields: ‘CPU-COMPANY-SYSTEM’ (e. --with-bash-malloc Use the Bash version of malloc in the directory ‘lib/malloc’. ‘configure --help’ prints the complete list. you can create a site shell script called config.
10.
10. where feature indicates an optional part of Bash. use ‘--disable-feature’.

--with-purify Define this to use the Purify memory allocation checker from Rational Software.6 [Aliases]. Bash links with the version in ‘lib/readline’. This option is enabled by default. which are subdirectories of prefix by default. This malloc is very fast. --with-curses Use the curses library instead of the termcap library. --with-installed-readline[=PREFIX] Define this to make Bash link with a locally-installed version of Readline rather than the version in ‘lib/readline’.0 and later versions. configure treats it as a directory pathname and looks for the installed version of Readline in subdirectories of that directory (include files in PREFIX /include and the library in PREFIX /lib). to find the installed version of Readline if it is not in the standard system include and library directories. unless the operating system does not provide the necessary support. but it is processed first. --enable-minimal-config This produces a shell with minimal features. All of the following options except for ‘disabled-builtins’ and ‘xpg-echo-default’ are enabled by default. close to the historical Bourne shell. If PREFIX is set to any other value. The ‘minimal-config’ option can be used to disable all of the following options. --with-gnu-malloc A synonym for --with-bash-malloc. so individual options may be enabled using ‘enable-feature’. --enable-alias Allow alias expansion and include the alias and unalias builtins (see Section 6. There are several ‘--enable-’ options that alter how Bash is compiled and linked. This should be supplied if your system has an inadequate or incomplete termcap database.
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from the 4. If PREFIX is no. rather than changing run-time features. configure uses the values of the make variables includedir and libdir. if gcc is being used. --enable-profiling This builds a Bash binary that produces profiling information to be processed by gprof each time it is executed. The ‘NOTES’ file contains a list of systems for which this should be turned off. This could be used to build a version to use as root’s shell. page 79). If PREFIX is yes or not supplied. if the operating system provides large file support. --enable-largefile Enable support for large files if the operating system requires special compiler options to build programs which can access large files. and configure disables this option automatically for a number of systems.2 bsd malloc. This works only with Readline 5. --enable-static-link This causes Bash to be linked statically. but wastes some space on each allocation. This is enabled by default.

(see Section 3. for example. will have their values converted to uppercase upon assignment.Chapter 10: Installing Bash
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--enable-arith-for-command Include support for the alternate form of the for command that behaves like the C language for statement (see Section 3. (see Section 3. --enable-coprocesses Include support for coprocesses and the coproc reserved word (see Section 3. for details of the builtin and enable builtin commands. --enable-debugger Include support for the bash debugger (distributed separately).2 [Pipelines].2. page 123).b}c → bac bbc ).1 [Looping Constructs]. See Section 4.2. --enable-brace-expansion Include csh-like brace expansion ( b{a.4. --enable-directory-stack Include support for a csh-like directory stack and the pushd. --enable-cond-regexp Include support for matching POSIX regular expressions using the ‘=~’ binary operator in the [[ conditional command. popd. page 41.7 [Arrays]. --enable-casemod-expansion Include support for case-modifying word expansions.2. --enable-command-timing Include support for recognizing time as a reserved word and for displaying timing statistics for the pipeline following time (see Section 3. page 10).2 [Pipelines]. page 8). See Section 3.2 [Conditional Constructs].4.4.2. --enable-disabled-builtins Allow builtin commands to be invoked via ‘builtin xxx’ even after xxx has been disabled using ‘enable -n xxx’.2 [Conditional Constructs]. and dirs builtins (see Section 6. --enable-casemod-attributes Include support for case-modifying attributes in the declare builtin and assignment statements. This allows pipelines as well as shell builtins and functions to be timed. page 10).2.2 [Bash Builtins]. page 8).
. page 9).8 [The Directory Stack].3 [History Interaction]. --enable-cond-command Include support for the [[ conditional command. --enable-array-variables Include support for one-dimensional array shell variables (see Section 6. for a complete description.5.1 [Brace Expansion]. page 80). page 81). --enable-bang-history Include support for csh-like history substitution (see Section 9. Variables with the uppercase attribute. page 18.

--enable-extended-glob-default Set the default value of the extglob shell option described above under Section 4.2 [The Shopt Builtin]. page 10). $PS3.5. $PS2.5.2 [Conditional Constructs]. --enable-multibyte This enables support for multibyte characters if the operating system provides the necessary support.4.6 [Programmable Completion].3. See Section 6. this option has no effect. If this is enabled. page 24. page 41).132
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--enable-dparen-arithmetic Include support for the ((. and $PS4 prompt strings.6 [Process Substitution].6 [Redirections]. page 89).1 [Bash History Facilities].
. page 93). If Readline is not enabled. --enable-net-redirections This enables the special handling of filenames of the form /dev/tcp/host/port and /dev/udp/host/port when used in redirections (see Section 3.2.10 [The Restricted Shell]. when called as rbash. --enable-process-substitution This enables process substitution (see Section 3.1 [Pattern Matching]. --enable-job-control This enables the job control features (see Chapter 7 [Job Control].. --enable-restricted Include support for a restricted shell. page 23) if the operating system provides the necessary support. --enable-extended-glob Include support for the extended pattern matching features described above under Section 3. page 121).8. --enable-history Include command history and the fc and history builtin commands (see Section 9.9 [Printing a Prompt]. for a description of restricted mode. --enable-prompt-string-decoding Turn on the interpretation of a number of backslash-escaped characters in the $PS1. page 115). for a complete list of prompt string escape sequences. See Section 6.)) command (see Section 3. which displays help on shell builtins and variables (see Section 4.2 [Bash Builtins]. --enable-progcomp Enable the programmable completion facilities (see Section 8. if the operating system supports them. Bash.. page 26). page 84. page 55 to be enabled. --enable-help-builtin Include the help builtin. enters a restricted mode. --enable-readline Include support for command-line editing and history with the Bash version of the Readline library (see Chapter 8 [Command Line Editing]. page 82.

The file ‘config-top. page 10). See Section 4.h’ contains C Preprocessor ‘#define’ statements for options which are not settable from configure. This sets the default value of the xpg_echo shell option to on.2.2 [Conditional Constructs]. for a description of the escape sequences that echo recognizes. This aids in translating the text to different languages. --enable-separate-helpfiles Use external files for the documentation displayed by the help builtin instead of storing the text internally.2 [Bash Builtins]. beware of the consequences if you do. Read the comments associated with each definition for more information about its effect.
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--enable-select Include the select builtin. --enable-single-help-strings Store the text displayed by the help builtin as a single string for each help topic. --enable-usg-echo-default A synonym for --enable-xpg-echo-default. You may need to disable this if your compiler cannot handle very long string literals. version 3. --enable-xpg-echo-default Make the echo builtin expand backslash-escaped characters by default. --enable-strict-posix-default Make Bash posix-conformant by default (see Section 6. page 41. which allows the generation of simple menus (see Section 3.11 [Bash POSIX Mode]. page 84). Some of these are not meant to be changed.4. which makes the Bash echo behave more like the version specified in the Single Unix Specification. without requiring the ‘-e’ option.

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Appendix A: Reporting Bugs
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Appendix A Reporting Bugs
Please report all bugs you find in Bash. • A description of the bug behaviour. and that it appears in the latest version of Bash. • A short script or ‘recipe’ which exercises the bug and may be used to reproduce it. But first. The latest version of Bash is always available for FTP from ftp://ftp. use the bashbug command to submit a bug report. All bug reports should include: • The version number of Bash.org or posted to the Usenet newsgroup gnu. you should make sure that it really is a bug. Please send all reports concerning this manual to chet.org/pub/gnu/bash/.bash.
.edu.ramey@case. • The hardware and operating system. Once you have determined that a bug actually exists. bashbug inserts the first three items automatically into the template it provides for filing a bug report. you are encouraged to mail that as well! Suggestions and ‘philosophical’ bug reports may be mailed to bug-bash@gnu. • The compiler used to compile Bash.bug. If you have a fix.gnu.

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expr3 )) arithmetic for command. page 84).1. ‘--dump-strings’. and compopt. • Bash implements the for (( expr1 . • Bash has multi-character invocation options (see Section 6.3 [History Interaction].4 [ANSI-C Quoting]. Several of the Bash builtins take options to act on arrays..1 [Bash History Facilities]. similar to the C language (see Section 3. The display of the timing statistics may be controlled with the TIMEFORMAT variable. The Bash ‘-o pipefail’ option to set will cause a pipeline to return a failure status if any command fails. There are some differences between the traditional Bourne shell and Bash. which allows the generation of simple menus (see Section 3.2 (the last version of the historical Bourne shell) as the baseline reference. This section uses the version of sh included in SVR4..’ quoting syntax. this section quickly details the differences of significance. and the appropriate variable expansions and assignment syntax to use them. page 10). even where the posix specification differs from traditional sh behavior (see Section 6. The Bash history list maintains timestamp information and uses the value of the HISTTIMEFORMAT variable to display it. page 8). compgen.4.4. • Bash has one-dimensional array variables (see Section 6. page 123).
. and ‘--dump-po-strings’ invocation options list the translatable strings found in a script (see Section 3. A number of these differences are explained in greater depth in previous sections. page 121) and the history and fc builtins to manipulate it..2 [Pipelines]. page 115). expr2 . Very useful when an if statement needs to act only if a test fails..7 [Arrays]. page 6). Bash provides a number of built-in array variables.1 [Invoking Bash].2. • Bash provides a programmable word completion mechanism (see Section 8. parameter and variable expansion.2. • Bash is posix-conformant. page 9).2.2. • Bash includes the select compound command.2 [Pipelines].Appendix B: Major Differences From The Bourne Shell
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Appendix B Major Differences From The Bourne Shell
Bash implements essentially the same grammar.1.2.6 [Programmable Completion].5 [Locale Translation]. • Bash has command-line editing (see Chapter 8 [Command Line Editing]. page 93) and the bind builtin. Bash uses the posix standard as the specification of how these features are to be implemented. page 71). which expands ANSI-C backslash-escaped characters in the text between the single quotes. • Bash supports the $". The ‘-D’. • Bash has the time reserved word and command timing (see Section 3. is supported (see Section 3. • Bash implements the ! keyword to negate the return value of a pipeline (see Section 3. to manipulate it. redirection.2." quoting syntax to do locale-specific translation of the characters between the double quotes. page 8).1 [Looping Constructs]. • The $’. and quoting as the Bourne Shell. • Bash has command history (see Section 9. and builtin commands complete. page 7). • Bash implements csh-like history expansion (see Section 9. page 80).2 [Conditional Constructs].11 [Bash POSIX Mode].

3. • The command builtin allows selective disabling of functions when command lookup is performed (see Section 4. The ‘>|’ redirection operator may be used to override noclobber. page 26). page 41). • The noclobber option is available to avoid overwriting existing files with output redirection (see Section 4.6 [Redirections].2 [Bash Builtins]. page 51). page 24). • Bash performs filename expansion on filenames specified as operands to input and output redirection operators (see Section 3.Appendix B: Major Differences From The Bourne Shell
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• Bash implements the full set of posix filename expansion operators. page 41).
. including character classes.1 [The Set Builtin]. equivalence classes. • Bash functions are permitted to have local variables using the local builtin. and collating symbols (see Section 3. page 26). • Shell functions may be exported to children via the environment using export -f (see Section 3. • The Bash exec builtin takes additional options that allow users to control the contents of the environment passed to the executed command. • Variable assignments preceding commands affect only that command. • It is possible to have a variable and a function with the same name. all variable assignments preceding commands are global unless the command is executed from the file system. page 14).5.2 [Bash Builtins]. • Bash contains the ‘<>’ redirection operator. even builtins and functions (see Section 3. • Bash implements extended pattern matching features when the extglob shell option is enabled (see Section 3. allowing a file to be opened for both reading and writing. • Bash allows a function to override a builtin with the same name.8. • Individual builtins may be enabled or disabled using the enable builtin (see Section 4. page 26). for directing standard output and standard error to the same file (see Section 3.2 [Bash Builtins]. In sh. page 41).6 [Redirections]. • Bash treats a number of filenames specially when they are used in redirection operators (see Section 3.2 [Bash Builtins].6 [Redirections].1 [Pattern Matching].1 [Bourne Shell Builtins]. sh does not separate the two name spaces.7.3 [Shell Functions].5. page 26). and what the zeroth argument to the command is to be (see Section 4.8 [Filename Expansion].6 [Redirections]. which move one file descriptor to another. and thus useful recursive functions may be written (see Section 4. • The Bash cd and pwd builtins (see Section 4. and the ‘&>’ redirection operator. page 31). • Bash includes the ‘<<<’ redirection operator. page 35) each take ‘-L’ and ‘-P’ options to switch between logical and physical modes.4 [Environment]. page 24). page 41). • Bash can open network connections to arbitrary machines and services with the redirection operators (see Section 3. and provides access to that builtin’s functionality within the function via the builtin and command builtins (see Section 4. page 35). • Bash implements the ‘[n]<&word’ and ‘[n]>&word’ redirection operators. allowing a string to be used as the standard input to a command.1 [Bourne Shell Builtins].

1 [Invoking Bash]. • The Bash hash builtin allows a name to be associated with an arbitrary filename.1 [Bourne Shell Builtins]. The trap builtin (see Section 4. page 41) will read a line ending in ‘\’ with the ‘-r’ option. page 35) is slightly different.2 [Bash Builtins].1 [Bourne Shell Builtins]. similar to EXIT and DEBUG. and declare builtins can take a ‘-f’ option to act on shell functions.1 [Bourne Shell Builtins].3.1 [The Set Builtin]. case command. page 41).1 [The Set Builtin]. • Bash has much more optional behavior controllable with the set builtin (see Section 4. for command. The extdebug shell option has additional effects on the DEBUG trap. • The trap builtin (see Section 4.2 [Bash Builtins]. every arithmetic for command. and before the first command executes in a shell function. • Bash includes the shopt builtin. even when that filename cannot be found by searching the $PATH.1 [Bourne Shell Builtins]. This supports the bash debugger. page 71). Commands specified with an ERR trap
. the ‘-n’ option will allow reading only a specified number of characters rather than a full line.2 [Bash Builtins].3. and the ‘-d’ option will read until a particular character rather than newline. page 35). page 35) allows an ERR pseudosignal specification. • The test builtin (see Section 4. readonly. and allows these options to be set and unset at shell invocation (see Section 6. and will use the REPLY variable as a default if no non-option arguments are supplied. page 55). page 51). or source builtins). • Bash includes the caller builtin. similar to EXIT. The Bash read builtin also accepts a prompt string with the ‘-p’ option and will use Readline to obtain the line when given the ‘-e’ option.3. • The ‘-x’ (xtrace) option displays commands other than simple commands when performing an execution trace (see Section 4. using ‘hash -p’ (see Section 4. • The printf builtin is available to display formatted output (see Section 4. the ‘-t’ option will allow read to time out if input does not arrive within a specified number of seconds. which specifies the behavior based on the number of arguments.2 [The Shopt Builtin]. for finer control of shell optional capabilities (see Section 4. • The return builtin may be used to abort execution of scripts executed with the . and ‘name=value’ arguments to set variable attributes and values simultaneously. page 41). page 35) allows a DEBUG pseudo-signal specification. select command. or source builtins (see Section 4. as it implements the posix algorithm. page 51). • Bash includes a help builtin for quick reference to shell facilities (see Section 4.140
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• The Bash export. The DEBUG trap is not inherited by shell functions unless the function has been given the trace attribute or the functrace option has been enabled using the shopt builtin. • The Bash read builtin (see Section 4. page 35).1 [Bourne Shell Builtins]. a ‘-p’ option to display variables with various attributes set in a format that can be used as shell input. Commands specified with a DEBUG trap are executed before every simple command. a ‘-n’ option to remove various variable attributes. which displays the context of any active subroutine call (a shell function or a script executed with the . The read builtin also has additional options to control input: the ‘-s’ option will turn off echoing of input characters as they are read.

Commands specified with an RETURN trap are executed before execution resumes after a shell function or a shell script executed with . The SVR4. • Bash does not allow unbalanced quotes.2 [Bash Builtins].9 [Printing a Prompt]. For instance: • Bash does not fork a subshell when redirecting into or out of a shell control structure such as an if or while statement. page 81). The trap builtin (see Section 4. with a few exceptions. If the shell is started from a process with SIGSEGV blocked (e. and provides the pushd. The Bash type builtin is more extensive and gives more information about the names it finds (see Section 4. Bash implements a csh-like directory stack.
B.1 [Bourne Shell Builtins]. similar to EXIT and DEBUG. Bash also makes the directory stack visible as the value of the DIRSTACK shell variable. Bash does not use the SHACCT variable or perform shell accounting.10 [The Restricted Shell]. page 71. page 84). popd..2 shell uses a baroque memory management scheme based on trapping SIGSEGV. it misbehaves badly. by using the system() C library function call).1 [Bourne Shell Builtins].1 Implementation Differences From The SVR4. page 35) allows a RETURN pseudo-signal specification.2 shell has two privilege-related builtins (mldmode and priv) not present in Bash. The Bash umask builtin permits a ‘-p’ option to cause the output to be displayed in the form of a umask command that may be reused as input (see Section 4. page 35).2 sh uses a TIMEOUT variable like Bash uses TMOUT. it does not suffer from many of the limitations of the SVR4. Bash interprets special backslash-escaped characters in the prompt strings when interactive (see Section 6.
. This can be the cause of some hardto-find errors. The SVR4.2 shell.8 [The Directory Stack]. page 41). page 82). The SVR4. page 90) or suppress the sending of SIGHUP to a job when the shell exits as the result of a SIGHUP. The RETURN trap is not inherited by shell functions unless the function has been given the trace attribute or the functrace option has been enabled using the shopt builtin.2 Shell
Since Bash is a completely new implementation. The Bash restricted mode is more useful (see Section 6. Bash does not have the stop or newgrp builtins. the SVR4.Appendix B: Major Differences From The Bourne Shell
141
• •
•
• • •
• • • • •
are executed after a simple command fails.
More features unique to Bash may be found in Chapter 6 [Bash Features].2 shell restricted mode is too limited.2 [Job Control Builtins].g.2 shell will silently insert a needed closing quote at EOF under certain circumstances. • The SVR4. and dirs builtins to manipulate it (see Section 6. The disown builtin can remove a job from the internal shell job table (see Section 7. Bash includes a number of features to support a separate debugger for shell scripts. The ERR trap is not inherited by shell functions unless the -o errtrace option to the set builtin is enabled. or source returns.

the SVR4. • The SVR4.142
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• In a questionable attempt at security.2 shell does not allow users to trap SIGSEGV. commonly 100. when invoked without the ‘-p’ option. In fact. PS1. or SIGCHLD. • Bash allows multiple option arguments when it is invoked (-x -v).2 shell behaves differently when invoked as jsh (it turns on job control). will alter its real and effective uid and gid if they are less than some magic threshold value. some versions of the shell dump core if the second argument begins with a ‘-’. MAILCHECK. • The SVR4. • The SVR4.
.2 shell treats ‘^’ as the undocumented equivalent of ‘|’. PATH.2 shell allows only one option argument (-xv). This can lead to unexpected results.2 shell. Bash exits a script only if one of the posix special builtins fails. and only for certain failures. as enumerated in the posix standard. • The SVR4.2 shell exits a script if any builtin fails. the SVR4. SIGALRM. or PS2 variables to be unset. • The SVR4.2 shell does not allow the IFS.

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